# BR-64 Item standard identifier must have a scheme identifier

The Item standard identifier (BT-157) shall have a Scheme identifier so the identifier’s registration scheme is unambiguous.

## Problem Description

The Item standard identifier (BT-157) shall have a Scheme identifier so the identifier’s registration scheme is unambiguous.

> **ERROR**: XRechnung 3.0

## Context and Significance

This rule comes from the EN16931 core semantic model enforced by the KoSIT XRechnung 3.0.2 Schematron, ensuring invoices carry the mandatory data needed for automated processing.

## Solution

Set the schemeID attribute on cac:StandardItemIdentification/cbc:ID using an ISO/IEC 6523 code, for example 0160 for GTIN.

- validate
- convert

## Valid Examples

- BT-157 = 04012345678901 with schemeID="0160" (GTIN)

## Invalid Examples

- BT-157 standard item ID present but with no schemeID attribute

## 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>04012345678901</cbc:ID>
  </cac:StandardItemIdentification>
</cac:Item>
```

## Affected Fields

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

## Related Rules

- [BR-65](/resources/kosit-validator-errors/br-65-item-classification-identifier-scheme-required)

## 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.