What it means
An invoice can point to the contract it belongs to with a contract reference (BT-12). The rule counts the contract identifiers in the file and allows at most one. In UBL that is the cbc:ID inside cac:ContractDocumentReference, so a second block with its own ID breaks the rule as well. It applies to invoices and credit notes alike.
Why it matters
EN 16931 has room for one contract reference only.
How to fix it
Open the invoice in your invoicing software and find the contract number or contract reference field. Make sure it holds one number, and that the same number is not also entered in a second field that is exported as a contract reference. If you need to mention more than one contract, put the extras in the free-text invoice note. If you build the XML yourself, keep one cac:ContractDocumentReference with one cbc:ID.
Once the file is corrected, check it again for free: Validate an invoice against EN 16931
Or fix the invoice in a form and download the corrected file, for 1 credit: Fix a rejected e-invoice
The rule as published
Contract identifier shall occur maximum once. Where this rule applies
| Rule set | Version |
|---|---|
| EN 16931 (UBL) | 1.3.17 |
| EN 16931 (UBL credit note) | 1.3.17 |
| XRechnung 3.0 (UBL) | 3.0.2 |
| XRechnung 3.0 (UBL credit note) | 3.0.2 |
| Peppol BIS Billing 3.0 (invoice) | 2026.5 |
| Peppol BIS Billing 3.0 (credit note) | 2026.5 |
| SI-UBL 2.0 / NLCIUS (invoice) | 2.0.3.13 |
| SI-UBL 2.0 / NLCIUS (credit note) | 2.0.3.13 |
| France CTC (UBL invoice) | 1.4.0-04 |
| France CTC (UBL credit note) | 1.4.0-04 |
| CIUS-RO UBL Invoice 1.0.9 | 1.0.9 |
| CIUS-RO UBL Credit Note 1.0.9 | 1.0.9 |
Check your file
Drop your e-invoice to see whether UBL-SR-01 still fails, and which other rules do.
Related rules
- UBL-SR-02 An invoice can have only one receiving advice reference
- UBL-SR-03 An invoice can have only one despatch advice reference
- UBL-SR-04 An invoice can have only one invoiced object identifier
- UBL-SR-05 Payment terms can be given only once
- UBL-SR-06 A preceding invoice reference can name only one invoice
- UBL-SR-07 A preceding invoice reference needs the invoice number