# UBL-SR-01

> Contract identifier shall occur maximum once.

Checked against Peppol BIS Billing 3.0 2026.5

Canonical: https://einvoicecheck.eu/rules/ubl-sr-01

## Official rule text

Contract identifier shall occur maximum once.

## Why did my invoice fail UBL-SR-01?

The Contract reference identifier (BT-12) may occur at most once on the invoice, so only a single cac:ContractDocumentReference is allowed at document level. EN 16931 defines the contract reference as a single-cardinality business term identifying the underlying contract.

## Common causes

- Two or more cac:ContractDocumentReference elements appear on the invoice.
- Contract numbers from multiple related agreements were all mapped to ContractDocumentReference.
- A duplicate contract reference was emitted during document assembly or merging.

## How do I fix it?

```
Keep exactly one: <cac:ContractDocumentReference><cbc:ID>CONTRACT-2026-88</cbc:ID></cac:ContractDocumentReference> and remove additional occurrences.
```