# BR-DE-21 Specification identifier must be provided

The specification identifier must be provided on the invoice.

## Problem Description

The specification identifier must be provided on the invoice.

> **ERROR**: XRechnung 3.0

## Context and Significance

The specification identifier (often CustomizationID) tells the receiver which exact rule set to apply (for example, a specific XRechnung version/profile). Without it, validators cannot reliably select the correct schematron rules.

## Solution

Add the specification identifier to the invoice.

- validate
- convert

## Valid Examples

- A valid CustomizationID/specification identifier required by the receiver and XRechnung version

## Invalid Examples

- Missing specification identifier (CustomizationID)

## Code Examples

### Valid XML
```
<cbc:CustomizationID>CUSTOMIZATION_ID_FROM_SPEC</cbc:CustomizationID>
```

### Invalid XML
```
<!-- Missing cbc:CustomizationID -->
```

## Technical Details

### Affected Fields
Specification Identifier (CustomizationID)

## Related Rules

- [BR-DE-16](/resources/kosit-validator-errors/br-de-16-business-process-type-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.