# BR-01 Specification identifier is required

An invoice shall carry a Specification identifier (BT-24) that names the syntax and business rules the document follows.

## Problem Description

An invoice shall carry a Specification identifier (BT-24) that names the syntax and business rules the document follows.

> **ERROR**: XRechnung 3.0

## Context and Significance

This is a core EN16931 rule; the identifier tells the validator which rule set to apply, so it must be present before any other check can run.

## Solution

Add cbc:CustomizationID with the EN16931/XRechnung specification URN, for example urn:cen.eu:en16931:2017#compliant#urn:xoev-de:kosit:standard:xrechnung_3.0.

- validate
- convert

## Valid Examples

- BT-24 = urn:cen.eu:en16931:2017#compliant#urn:xoev-de:kosit:standard:xrechnung_3.0

## Invalid Examples

- cbc:CustomizationID element is missing entirely from the document

## Code Examples

### Valid XML
```
<cbc:CustomizationID>urn:cen.eu:en16931:2017#compliant#urn:xoev-de:kosit:standard:xrechnung_3.0</cbc:CustomizationID>
<cbc:ID>RE-2024-0001</cbc:ID>
```

### Invalid XML
```
<cbc:ID>RE-2024-0001</cbc:ID>
<cbc:IssueDate>2024-05-01</cbc:IssueDate>
```

## Affected Fields

- [BT-24 Specification identifier](/resources/xrechnung/bt-24-specification-identifier)

## Related Rules

- [BR-DE-21](/resources/kosit-validator-errors/br-de-21-specification-identifier-syntax)

## 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.