# BR-CL-07 Object identifier scheme must come from UNTDID 1153

The object identifier scheme identifier (BT-18-1) must be a code from the UNTDID 1153 restriction used for referenced objects.

## Problem Description

The object identifier scheme identifier (BT-18-1) must be a code from the UNTDID 1153 restriction used for referenced objects.

> **ERROR**: XRechnung 3.0

## Context and Significance

This EN16931 core code-list rule keeps the type of the referenced object (e.g. contract, order) machine-identifiable.

## Solution

Set the schemeID of BT-18 to a valid UNTDID 1153 qualifier such as AAK (order number) or AWV; do not invent scheme codes.

- validate
- convert

## Valid Examples

- BT-18-1 = AAK — object identifier for an order
- schemeID present and part of UNTDID 1153

## Invalid Examples

- BT-18-1 = ZZ9 — not a UNTDID 1153 code
- BT-18-1 = ORDER — free text scheme

## Code Examples

### Valid XML
```
<cac:AdditionalDocumentReference>
  <cbc:ID schemeID="AAK">PO-4711</cbc:ID>
  <cbc:DocumentTypeCode>130</cbc:DocumentTypeCode>
</cac:AdditionalDocumentReference>
```

### Invalid XML
```
<cac:AdditionalDocumentReference>
  <cbc:ID schemeID="ZZ9">PO-4711</cbc:ID>
  <cbc:DocumentTypeCode>130</cbc:DocumentTypeCode>
</cac:AdditionalDocumentReference>
```

## Affected Fields

- [BT-18-1 Object identifier scheme identifier](/resources/xrechnung/bt-18-invoiced-object-identifier)

## Related Rules

- [BR-CL-10](/resources/kosit-validator-errors/br-cl-10-iso-6523-any-identifier-scheme)
- [BR-CL-11](/resources/kosit-validator-errors/br-cl-11-iso-6523-registration-scheme)

## 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.