Send UBL documents

Overview

If your system generates UBL XML files, you can send them through e-invoice.be without a conversion to JSON. Use this method when:

Your ERP system generates UBL XML.
You migrate from a different Peppol Access Point.
You have existing UBL files to send.
You want full control of the UBL structure.

If your ERP can only send invoices by email, see Send invoices by email (Mailbox).

Prerequisites

Your UBL XML must:

Be in the Peppol BIS Billing 3.0 format.
Be an invoice (Invoice) or a credit note (CreditNote).
Have your company as the supplier. The send request returns 409 if the sender Peppol ID is not one of the peppol_ids of your company.

The three steps are:

Validate the UBL XML (recommended while you develop).
Create the document from the UBL file.
Send the document.

POST /api/validate/ubl and POST /api/documents/ubl accept multipart/form-data with one form field with the name file. A raw XML body (Content-Type: application/xml) gives 422 with the message Field required for body.file.

Validate the UBL XML

Validation does not create a document. POST /api/validate/ubl accepts only a file part with the content type application/xml or text/xml.

cURL

curl -X POST "https://api.e-invoice.be/api/validate/ubl" \
  -H "Authorization: Bearer $E_INVOICE_API_KEY" \
  -F "file=@invoice.xml;type=application/xml"

The validation guide has this request in Node.js, Python, PHP and C#. Response for a valid file (201):

{
  "id": "a1b2c3d4-5678-90ab-cdef-1234567890ab",
  "file_name": "invoice.xml",
  "is_valid": true,
  "issues": []
}

Response for a file that is not valid (201, is_valid is false):

{
  "id": "a1b2c3d4-5678-90ab-cdef-1234567890ab",
  "file_name": "invoice.xml",
  "is_valid": false,
  "issues": [
    {
      "message": "Belgian enterprise number MUST be stated in the correct format.",
      "type": "error",
      "rule_id": "PEPPOL-COMMON-R043",
      "flag": "fatal",
      "schematron": "PEPPOL-EN16931-UBL"
    }
  ]
}

Create the document from UBL

POST /api/documents/ubl validates the file again and creates the document in state DRAFT. If the file has a fatal validation issue, the request returns 415 and no issue list. Thus use POST /api/validate/ubl to find the cause.

curl -X POST "https://api.e-invoice.be/api/documents/ubl" \
  -H "Authorization: Bearer $E_INVOICE_API_KEY" \
  -F "file=@invoice.xml;type=application/xml"

import { readFile } from "node:fs/promises";

const form = new FormData();
form.append(
  "file",
  new Blob([await readFile("invoice.xml")], { type: "application/xml" }),
  "invoice.xml",
);

// Do not set the Content-Type header. fetch adds it with the multipart boundary.
const response = await fetch("https://api.e-invoice.be/api/documents/ubl", {
  method: "POST",
  headers: { Authorization: `Bearer ${process.env.E_INVOICE_API_KEY}` },
  body: form,
});

const document = await response.json();
if (!response.ok) {
  throw new Error(`HTTP ${response.status}: ${JSON.stringify(document)}`);
}
console.log(document.id, document.state);

import os

import requests

# Do not set the Content-Type header. requests adds it with the multipart boundary.
with open("invoice.xml", "rb") as f:
    response = requests.post(
        "https://api.e-invoice.be/api/documents/ubl",
        headers={"Authorization": f"Bearer {os.environ['E_INVOICE_API_KEY']}"},
        files={"file": ("invoice.xml", f, "application/xml")},
        timeout=60,
    )

response.raise_for_status()
document = response.json()
print(document["id"], document["state"])

<?php
$ch = curl_init("https://api.e-invoice.be/api/documents/ubl");
curl_setopt_array($ch, [
    CURLOPT_POST => true,
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_HTTPHEADER => ["Authorization: Bearer " . getenv("E_INVOICE_API_KEY")],
    // An array as body makes the request multipart/form-data.
    CURLOPT_POSTFIELDS => [
        "file" => new CURLFile("invoice.xml", "application/xml", "invoice.xml"),
    ],
]);

$body = curl_exec($ch);
$status = curl_getinfo($ch, CURLINFO_HTTP_CODE);
curl_close($ch);

if ($status !== 201) {
    throw new RuntimeException("HTTP $status: $body");
}
$document = json_decode($body, true);
echo $document["id"] . " " . $document["state"] . PHP_EOL;

using System.Net.Http.Headers;

using var client = new HttpClient();
client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue(
    "Bearer", Environment.GetEnvironmentVariable("E_INVOICE_API_KEY"));

var file = new ByteArrayContent(await File.ReadAllBytesAsync("invoice.xml"));
file.Headers.ContentType = new MediaTypeHeaderValue("application/xml");

using var form = new MultipartFormDataContent();
form.Add(file, "file", "invoice.xml");

var response = await client.PostAsync("https://api.e-invoice.be/api/documents/ubl", form);
var body = await response.Content.ReadAsStringAsync();
if (!response.IsSuccessStatusCode)
{
    throw new HttpRequestException($"HTTP {(int)response.StatusCode}: {body}");
}
Console.WriteLine(body);

Response (201, shortened; the full response also has the line items, the tax details and the attachments):

{
  "id": "doc-1a2b3c4d5e6f7a8b9c0d1e2f3a4b5c6d",
  "created_at": "2026-10-01T12:00:00Z",
  "document_type": "INVOICE",
  "state": "DRAFT",
  "direction": "OUTBOUND",
  "invoice_id": "INV-2026-001",
  "invoice_date": "2026-10-01",
  "due_date": "2026-10-31",
  "currency": "EUR",
  "vendor_name": "E-INVOICE BV",
  "vendor_tax_id": "BE1018265814",
  "customer_name": "OpenPeppol VZW",
  "customer_tax_id": "BE0848934496",
  "subtotal": "1000.00",
  "total_tax": "210.00",
  "invoice_total": "1210.00"
}

Files that are embedded in the UBL become attachments of the document. See Attachments and PDF.

Send the document

Send the document with the id from the create response. The samples read the ID from the environment variable DOCUMENT_ID.

curl -X POST "https://api.e-invoice.be/api/documents/$DOCUMENT_ID/send?sender_peppol_scheme=0208&sender_peppol_id=1018265814&receiver_peppol_scheme=0208&receiver_peppol_id=0848934496" \
  -H "Authorization: Bearer $E_INVOICE_API_KEY"

const documentId = process.env.DOCUMENT_ID; // the "id" from the create response

const query = new URLSearchParams({
  sender_peppol_scheme: "0208",
  sender_peppol_id: "1018265814",
  receiver_peppol_scheme: "0208",
  receiver_peppol_id: "0848934496",
});

const response = await fetch(
  `https://api.e-invoice.be/api/documents/${documentId}/send?${query}`,
  {
    method: "POST",
    headers: { Authorization: `Bearer ${process.env.E_INVOICE_API_KEY}` },
  },
);

const result = await response.json();
if (!response.ok) {
  throw new Error(`HTTP ${response.status}: ${JSON.stringify(result)}`);
}
console.log(result.state);

import os

import requests

document_id = os.environ["DOCUMENT_ID"]  # the "id" from the create response

response = requests.post(
    f"https://api.e-invoice.be/api/documents/{document_id}/send",
    headers={"Authorization": f"Bearer {os.environ['E_INVOICE_API_KEY']}"},
    params={
        "sender_peppol_scheme": "0208",
        "sender_peppol_id": "1018265814",
        "receiver_peppol_scheme": "0208",
        "receiver_peppol_id": "0848934496",
    },
    timeout=30,
)
if not response.ok:
    raise SystemExit(f"HTTP {response.status_code}: {response.text}")
print(response.json()["state"])

<?php
$documentId = getenv("DOCUMENT_ID"); // the "id" from the create response

$query = http_build_query([
    "sender_peppol_scheme" => "0208",
    "sender_peppol_id" => "1018265814",
    "receiver_peppol_scheme" => "0208",
    "receiver_peppol_id" => "0848934496",
]);

$ch = curl_init("https://api.e-invoice.be/api/documents/$documentId/send?$query");
curl_setopt_array($ch, [
    CURLOPT_POST => true,
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_HTTPHEADER => [
        "Authorization: Bearer " . getenv("E_INVOICE_API_KEY"),
    ],
]);

$body = curl_exec($ch);
$status = curl_getinfo($ch, CURLINFO_HTTP_CODE);
curl_close($ch);

if ($status >= 400) {
    throw new RuntimeException("HTTP $status: $body");
}
echo json_decode($body, true)["state"], PHP_EOL;

using System.Net.Http.Headers;

var documentId = Environment.GetEnvironmentVariable("DOCUMENT_ID"); // the "id" from the create response

using var client = new HttpClient { BaseAddress = new Uri("https://api.e-invoice.be") };
client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue(
    "Bearer", Environment.GetEnvironmentVariable("E_INVOICE_API_KEY"));

var query = "sender_peppol_scheme=0208&sender_peppol_id=1018265814&receiver_peppol_scheme=0208&receiver_peppol_id=0848934496";

var response = await client.PostAsync($"/api/documents/{documentId}/send?{query}", null);

var body = await response.Content.ReadAsStringAsync();
if (!response.IsSuccessStatusCode)
{
    throw new HttpRequestException($"HTTP {(int)response.StatusCode}: {body}");
}
Console.WriteLine(body);

Response (200, shortened):

{
  "id": "doc-1a2b3c4d5e6f7a8b9c0d1e2f3a4b5c6d",
  "created_at": "2026-10-01T12:00:00Z",
  "document_type": "INVOICE",
  "state": "TRANSIT",
  "direction": "OUTBOUND",
  "invoice_id": "INV-2026-001"
}

To follow the document after the send, see Document lifecycle and delivery tracking.

Peppol ID routing

When you create a document from UBL, the API stores the cbc:EndpointID values of the supplier and the customer with the document. The send request uses these values if you do not give query parameters. When you send a document, the API finds the sender and receiver Peppol IDs in this order:

The query parameters of the send request.
The Peppol IDs that the API stored with the UBL of the document.
The identifiers in the document: vendor_tax_id or vendor_company_id for the sender, and customer_peppol_id, customer_tax_id or customer_company_id for the receiver.

For a Belgian party, the API derives scheme 0208 and the enterprise number: the tax ID BE1018265814 gives the Peppol ID 0208:1018265814. For other countries, the derived scheme can be different from the scheme that the receiver registered. Thus the best practice is to give all four query parameters:

cURL

curl -X POST "https://api.e-invoice.be/api/documents/$DOCUMENT_ID/send?sender_peppol_scheme=0208&sender_peppol_id=1018265814&receiver_peppol_scheme=0208&receiver_peppol_id=0848934496" \
  -H "Authorization: Bearer $E_INVOICE_API_KEY"

string

Scheme of the sender Peppol ID. Example: 0208.

string

Identifier of the sender, without the scheme. Example: 1018265814.

string

Scheme of the receiver Peppol ID. Example: 0208.

string

Identifier of the receiver, without the scheme. Example: 0848934496.

The API returns 400 if it cannot find a complete sender and receiver Peppol ID. It returns 409 if the sender Peppol ID is not one of the peppol_ids of your company.

Before you send, make sure that the receiver is registered on the Peppol network with the Peppol ID that you use. See Look up Peppol participants.

Example UBL invoice

This is a minimal invoice in the Peppol BIS Billing 3.0 format. Replace the supplier data with the data of your company.

<?xml version="1.0" encoding="UTF-8"?>
<Invoice xmlns="urn:oasis:names:specification:ubl:schema:xsd:Invoice-2"
         xmlns:cac="urn:oasis:names:specification:ubl:schema:xsd:CommonAggregateComponents-2"
         xmlns:cbc="urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2">

  <cbc:CustomizationID>urn:cen.eu:en16931:2017#compliant#urn:fdc:peppol.eu:2017:poacc:billing:3.0</cbc:CustomizationID>
  <cbc:ProfileID>urn:fdc:peppol.eu:2017:poacc:billing:01:1.0</cbc:ProfileID>

  <cbc:ID>INV-2026-001</cbc:ID>
  <cbc:IssueDate>2026-10-01</cbc:IssueDate>
  <cbc:DueDate>2026-10-31</cbc:DueDate>
  <cbc:InvoiceTypeCode>380</cbc:InvoiceTypeCode>
  <cbc:DocumentCurrencyCode>EUR</cbc:DocumentCurrencyCode>
  <cbc:BuyerReference>PO-12345</cbc:BuyerReference>

  <!-- Supplier -->
  <cac:AccountingSupplierParty>
    <cac:Party>
      <cbc:EndpointID schemeID="0208">1018265814</cbc:EndpointID>
      <cac:PartyName>
        <cbc:Name>E-INVOICE BV</cbc:Name>
      </cac:PartyName>
      <cac:PostalAddress>
        <cbc:StreetName>Brusselsesteenweg 119/A</cbc:StreetName>
        <cbc:CityName>Zemst</cbc:CityName>
        <cbc:PostalZone>1980</cbc:PostalZone>
        <cac:Country>
          <cbc:IdentificationCode>BE</cbc:IdentificationCode>
        </cac:Country>
      </cac:PostalAddress>
      <cac:PartyTaxScheme>
        <cbc:CompanyID>BE1018265814</cbc:CompanyID>
        <cac:TaxScheme>
          <cbc:ID>VAT</cbc:ID>
        </cac:TaxScheme>
      </cac:PartyTaxScheme>
      <cac:PartyLegalEntity>
        <cbc:RegistrationName>E-INVOICE BV</cbc:RegistrationName>
        <cbc:CompanyID schemeID="0208">1018265814</cbc:CompanyID>
      </cac:PartyLegalEntity>
    </cac:Party>
  </cac:AccountingSupplierParty>

  <!-- Customer -->
  <cac:AccountingCustomerParty>
    <cac:Party>
      <cbc:EndpointID schemeID="0208">0848934496</cbc:EndpointID>
      <cac:PartyName>
        <cbc:Name>OpenPeppol VZW</cbc:Name>
      </cac:PartyName>
      <cac:PostalAddress>
        <cbc:StreetName>Robert Schumanplein 6 bus 5</cbc:StreetName>
        <cbc:CityName>Brussel</cbc:CityName>
        <cbc:PostalZone>1040</cbc:PostalZone>
        <cac:Country>
          <cbc:IdentificationCode>BE</cbc:IdentificationCode>
        </cac:Country>
      </cac:PostalAddress>
      <cac:PartyTaxScheme>
        <cbc:CompanyID>BE0848934496</cbc:CompanyID>
        <cac:TaxScheme>
          <cbc:ID>VAT</cbc:ID>
        </cac:TaxScheme>
      </cac:PartyTaxScheme>
      <cac:PartyLegalEntity>
        <cbc:RegistrationName>OpenPeppol VZW</cbc:RegistrationName>
        <cbc:CompanyID schemeID="0208">0848934496</cbc:CompanyID>
      </cac:PartyLegalEntity>
    </cac:Party>
  </cac:AccountingCustomerParty>

  <!-- Tax total -->
  <cac:TaxTotal>
    <cbc:TaxAmount currencyID="EUR">210.00</cbc:TaxAmount>
    <cac:TaxSubtotal>
      <cbc:TaxableAmount currencyID="EUR">1000.00</cbc:TaxableAmount>
      <cbc:TaxAmount currencyID="EUR">210.00</cbc:TaxAmount>
      <cac:TaxCategory>
        <cbc:ID>S</cbc:ID>
        <cbc:Percent>21</cbc:Percent>
        <cac:TaxScheme>
          <cbc:ID>VAT</cbc:ID>
        </cac:TaxScheme>
      </cac:TaxCategory>
    </cac:TaxSubtotal>
  </cac:TaxTotal>

  <!-- Totals -->
  <cac:LegalMonetaryTotal>
    <cbc:LineExtensionAmount currencyID="EUR">1000.00</cbc:LineExtensionAmount>
    <cbc:TaxExclusiveAmount currencyID="EUR">1000.00</cbc:TaxExclusiveAmount>
    <cbc:TaxInclusiveAmount currencyID="EUR">1210.00</cbc:TaxInclusiveAmount>
    <cbc:PayableAmount currencyID="EUR">1210.00</cbc:PayableAmount>
  </cac:LegalMonetaryTotal>

  <!-- Invoice lines -->
  <cac:InvoiceLine>
    <cbc:ID>1</cbc:ID>
    <cbc:InvoicedQuantity unitCode="C62">10</cbc:InvoicedQuantity>
    <cbc:LineExtensionAmount currencyID="EUR">1000.00</cbc:LineExtensionAmount>
    <cac:Item>
      <cbc:Name>Professional services</cbc:Name>
      <cac:ClassifiedTaxCategory>
        <cbc:ID>S</cbc:ID>
        <cbc:Percent>21</cbc:Percent>
        <cac:TaxScheme>
          <cbc:ID>VAT</cbc:ID>
        </cac:TaxScheme>
      </cac:ClassifiedTaxCategory>
    </cac:Item>
    <cac:Price>
      <cbc:PriceAmount currencyID="EUR">100.00</cbc:PriceAmount>
    </cac:Price>
  </cac:InvoiceLine>

</Invoice>

UBL credit notes

A credit note uses the same three steps. The root element is CreditNote, and the type code element is cbc:CreditNoteTypeCode:

<?xml version="1.0" encoding="UTF-8"?>
<CreditNote xmlns="urn:oasis:names:specification:ubl:schema:xsd:CreditNote-2"
            xmlns:cac="urn:oasis:names:specification:ubl:schema:xsd:CommonAggregateComponents-2"
            xmlns:cbc="urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2">

  <cbc:CustomizationID>urn:cen.eu:en16931:2017#compliant#urn:fdc:peppol.eu:2017:poacc:billing:3.0</cbc:CustomizationID>
  <cbc:ProfileID>urn:fdc:peppol.eu:2017:poacc:billing:01:1.0</cbc:ProfileID>

  <cbc:ID>CN-2026-001</cbc:ID>
  <cbc:IssueDate>2026-10-15</cbc:IssueDate>
  <cbc:CreditNoteTypeCode>381</cbc:CreditNoteTypeCode>
  <cbc:DocumentCurrencyCode>EUR</cbc:DocumentCurrencyCode>

  <!-- Reference to the original invoice -->
  <cac:BillingReference>
    <cac:InvoiceDocumentReference>
      <cbc:ID>INV-2026-001</cbc:ID>
    </cac:InvoiceDocumentReference>
  </cac:BillingReference>

  <!-- The parties, totals and cac:CreditNoteLine elements go here -->

</CreditNote>

Complete example

The example validates a UBL file, creates the document and sends it. It needs only the environment variable E_INVOICE_API_KEY.

Validate, create and send a UBL file (Node.js and Python)

import { readFile } from "node:fs/promises";
import { basename } from "node:path";

const BASE_URL = "https://api.e-invoice.be";
const headers = { Authorization: `Bearer ${process.env.E_INVOICE_API_KEY}` };

async function call(path, options = {}) {
  const response = await fetch(`${BASE_URL}${path}`, { ...options, headers });
  const body = await response.json();
  if (!response.ok) {
    throw new Error(`HTTP ${response.status}: ${JSON.stringify(body)}`);
  }
  return body;
}

// Make a new FormData object for each request.
async function ublForm(filePath) {
  const form = new FormData();
  form.append(
    "file",
    new Blob([await readFile(filePath)], { type: "application/xml" }),
    basename(filePath),
  );
  return form;
}

async function sendUblInvoice(filePath) {
  // 1. Validate the UBL
  const validation = await call("/api/validate/ubl", {
    method: "POST",
    body: await ublForm(filePath),
  });
  if (!validation.is_valid) {
    console.error("UBL validation failed:", validation.issues);
    return null;
  }
  console.log("OK: UBL is valid");

  // 2. Create the document
  const document = await call("/api/documents/ubl", {
    method: "POST",
    body: await ublForm(filePath),
  });
  console.log("OK: document created:", document.id);

  // 3. Send the document
  const sent = await call(`/api/documents/${document.id}/send`, { method: "POST" });
  console.log("OK: document sent, state:", sent.state);
  return sent;
}

await sendUblInvoice("./invoices/invoice.xml");

import os

import requests

BASE_URL = "https://api.e-invoice.be"
HEADERS = {"Authorization": f"Bearer {os.environ['E_INVOICE_API_KEY']}"}


def upload(path, ubl_file_path):
    # requests makes the multipart body. Do not set the Content-Type header.
    with open(ubl_file_path, "rb") as f:
        response = requests.post(
            f"{BASE_URL}{path}",
            headers=HEADERS,
            files={"file": (os.path.basename(ubl_file_path), f, "application/xml")},
            timeout=60,
        )
    response.raise_for_status()
    return response.json()


def send_ubl_invoice(ubl_file_path):
    # 1. Validate the UBL
    validation = upload("/api/validate/ubl", ubl_file_path)
    if not validation["is_valid"]:
        print("UBL validation failed:", validation["issues"])
        return None
    print("OK: UBL is valid")

    # 2. Create the document
    document = upload("/api/documents/ubl", ubl_file_path)
    print("OK: document created:", document["id"])

    # 3. Send the document
    response = requests.post(
        f"{BASE_URL}/api/documents/{document['id']}/send",
        headers=HEADERS,
        timeout=60,
    )
    response.raise_for_status()
    sent = response.json()
    print("OK: document sent, state:", sent["state"])
    return sent


send_ubl_invoice("./invoices/invoice.xml")

Get the UBL of a document

GET /api/documents/{document_id}/ubl gives the UBL of a document, also for a document that you created from JSON. The response is JSON with a signed_url for the download of the XML file, not the XML itself.

cURL

curl "https://api.e-invoice.be/api/documents/$DOCUMENT_ID/ubl" \
  -H "Authorization: Bearer $E_INVOICE_API_KEY"

For the response and the lifetime of the URL, see Attachments and PDF.

Errors and frequent validation issues

Status	`detail`	Cause
`422`	`Field required` (`body.file`)	The request is not `multipart/form-data` with a `file` field.
`422`	`Invalid file type`	`POST /api/validate/ubl`: the content type of the file part is not `application/xml` or `text/xml`.
`415`	`UBL file is empty`	`POST /api/documents/ubl`: the file has no content.
`415`	`UBL file is not valid and contains errors`	`POST /api/documents/ubl`: the file has a fatal validation issue. Use `POST /api/validate/ubl` to get the issues.
`422`	`UBL File could not be parsed`	`POST /api/documents/ubl`: the API could not read the document data from the file.

Frequent causes of validation issues:

Profile ID: cbc:ProfileID must be urn:fdc:peppol.eu:2017:poacc:billing:01:1.0.
Customisation ID: cbc:CustomizationID must be urn:cen.eu:en16931:2017#compliant#urn:fdc:peppol.eu:2017:poacc:billing:3.0.
Endpoint ID: the supplier party and the customer party each need a cbc:EndpointID with a schemeID attribute, for example <cbc:EndpointID schemeID="0208">1018265814</cbc:EndpointID>.
Tax category: cac:TaxCategory/cbc:ID must be a code from the UNCL5305 list (S, E, Z, AE and others).
Namespaces: the root element must declare the three UBL namespaces that the example shows.

For all status codes and error formats, see Errors and troubleshooting.

Best practices

Validate with POST /api/validate/ubl while you develop. The create request rejects a file that is not valid, but it does not return the issues.
Give the four Peppol ID query parameters on the send request, and make sure that the receiver is registered before you send. See Look up Peppol participants.
Develop and test with a sandbox company. See Test mode and sandbox companies.
Keep a copy of the original UBL file for your audit records.

Getting started

Send documents

Receive and manage

Other input formats

Integrate

SDKs and tools

Resellers

Reference

Overview

Prerequisites

Validate the UBL XML

Create the document from UBL

Send the document

Peppol ID routing

Example UBL invoice

UBL credit notes

Complete example

Get the UBL of a document

Errors and frequent validation issues

Best practices

Resources

Next Steps

Create documents from PDF

Validation during development

Look up Peppol participants

Errors and troubleshooting

​Overview

​Prerequisites

​Validate the UBL XML

​Create the document from UBL

​Send the document

​Peppol ID routing

​Example UBL invoice

​UBL credit notes

​Complete example

​Get the UBL of a document

​Errors and frequent validation issues

​Best practices

​Resources

​Next Steps

Create documents from PDF

Validation during development

Look up Peppol participants

Errors and troubleshooting

Overview

Prerequisites

Validate the UBL XML

Create the document from UBL

Send the document

Peppol ID routing

Example UBL invoice

UBL credit notes

Complete example

Get the UBL of a document

Errors and frequent validation issues

Best practices

Resources

Next Steps