# BR-DE-19 Project reference must be provided

The project reference must be provided on the invoice.

## Problem Description

The project reference must be provided on the invoice.

> **ERROR**: XRechnung 3.0

## Context and Significance

Project references help the buyer allocate costs and match the invoice to a project or budget line. Use the project ID provided by the buyer (not an internal free-text label) where possible.

## Solution

Add the project reference to the invoice.

## Valid Examples

- PRJ-2025-0042
- Project ID provided by the buyer

## Invalid Examples

- Missing project reference

## Code Examples

### Valid XML
```
<cac:ProjectReference><cbc:ID>PRJ-2025-0042</cbc:ID></cac:ProjectReference>
```

### Invalid XML
```
<!-- Missing cac:ProjectReference -->
```

## Technical Details

### Affected Fields
Project Reference

## Related Rules

- [BR-DE-13](/resources/kosit-validator-errors/br-de-13-buyer-reference-must-be-provided)

## 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.