# 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

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

> **ERROR**: XRechnung 3.0

## Context and Significance

This rule comes from the EN16931 core semantic model enforced by the KoSIT XRechnung 3.0.2 Schematron, ensuring invoices carry the mandatory data needed for automated processing.

## Solution

Add cbc:ID to each cac:AdditionalDocumentReference that carries or references a supporting document.

- validate
- convert

## Valid Examples

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

## Invalid Examples

- Supporting document group with attachment but no BT-122 reference

## 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: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://erechnungsvalidator.service-bw.de/): Validate your XRechnung files against all BR-DE rules.
- [Convert PDF to XRechnung](/pdf-to-xrechnung): Automatic validation error fixes when converting.

## Related Resources

- [Official Specification](https://xeinkauf.de/xrechnung/)
- [Test Suite](https://github.com/itplr-kosit/xrechnung-testsuite)
- [Complete Error List](/resources/kosit-validator-errors): Overview of all German XRechnung validation rules.