# BR-52 Additional supporting document must have a reference

Each Additional supporting document (BG-24) shall contain a Supporting document reference (BT-122).

## Problem Description

> **ERROR**: EN 16931 core business rule (CEN)

## Context and Significance

This is an EN 16931 core rule. The reference (BT-122) lets the buyer match an attachment or external link, such as a timesheet, delivery note or statement, to the invoice. BR-52 checks that the reference is not empty; a missing cbc:ID in UBL is already rejected by the schema. The error appears when a file is attached or linked by URL but only a description or file name is exported.

## Solution

Fill cbc:ID in each cac:AdditionalDocumentReference (CII: ram:AdditionalReferencedDocument/ram:IssuerAssignedID) that carries or references a supporting document, for example with the timesheet or delivery note number.

- [Validate XML](/en/xrechnung-validator?source=resource&rule=BR-52)
- [Convert invoice](/en/pdf-to-xrechnung?source=resource&rule=BR-52)

## Valid Examples

- BT-122 = "TIMESHEET-2026-02" for an attached timesheet

## Invalid Examples

- cac:AdditionalDocumentReference with an empty cbc:ID and a description, or a CII ram:AdditionalReferencedDocument without ram:IssuerAssignedID; in UBL a missing cbc:ID is already an XML schema error

## Code Examples

### Valid XML
```
<cac:AdditionalDocumentReference>
  <cbc:ID>TIMESHEET-2026-02</cbc:ID>
  <cbc:DocumentDescription>Timesheet February 2026</cbc:DocumentDescription>
</cac:AdditionalDocumentReference>
```

### Invalid XML
```
<cac:AdditionalDocumentReference>
  <cbc:ID></cbc:ID>
  <cbc:DocumentDescription>Timesheet February 2026</cbc:DocumentDescription>
</cac:AdditionalDocumentReference>
```

## Affected Fields

- [BT-122 Supporting document reference](/resources/xrechnung/bt-122-supporting-document-reference)

## Related Rules

- [BR-55](/resources/kosit-validator-errors/br-55-preceding-invoice-reference-required)

## Validation Tools

- [Official KoSIT Validator](https://github.com/itplr-kosit/validator): Run the official KoSIT validator with the XRechnung configuration to check EN 16931 and XRechnung rules.
- [Convert PDF to XRechnung](/pdf-to-xrechnung): Find validation errors during conversion and review each correction.

## Related Resources

- [Official Specification](https://docs.peppol.eu/poacc/billing/3.0/rules/ubl-tc434/)
- [Test Suite](https://github.com/itplr-kosit/xrechnung-testsuite)
- [All validation errors](/resources/kosit-validator-errors): Overview of the EN 16931, XRechnung and Peppol validation rules explained here.