# BR-CL-21 Item standard identifier scheme must come from ISO 6523 ICD

The item standard identifier scheme identifier (BT-157-1) must be a code from the ISO 6523 ICD list.

## Problem Description

The item standard identifier scheme identifier (BT-157-1) must be a code from the ISO 6523 ICD list.

> **ERROR**: XRechnung 3.0

## Context and Significance

This EN16931 core code-list rule lets buyers resolve a standard item identifier (such as a GTIN) against its issuing scheme.

## Solution

Set the schemeID of BT-157 to a valid ISO 6523 ICD code, e.g. 0160 (GTIN/GS1) or 0088 (GLN).

- validate
- convert

## Valid Examples

- BT-157-1 = 0160 — GTIN issued by GS1
- schemeID present and part of ISO 6523 ICD

## Invalid Examples

- BT-157-1 = GTIN — name instead of ICD code
- BT-157-1 = EAN — not an ISO 6523 ICD value

## Code Examples

### Valid XML
```
<cac:Item>
  <cac:StandardItemIdentification>
    <cbc:ID schemeID="0160">04012345678901</cbc:ID>
  </cac:StandardItemIdentification>
</cac:Item>
```

### Invalid XML
```
<cac:Item>
  <cac:StandardItemIdentification>
    <cbc:ID schemeID="GTIN">04012345678901</cbc:ID>
  </cac:StandardItemIdentification>
</cac:Item>
```

## Affected Fields

- [BT-157-1 Item standard identifier scheme identifier](/resources/xrechnung/bt-157-item-standard-identifier)

## Related Rules

- [BR-CL-10](/resources/kosit-validator-errors/br-cl-10-iso-6523-any-identifier-scheme)
- [BR-CL-13](/resources/kosit-validator-errors/br-cl-13-untdid-7143-item-classification)

## 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.