# UBL-SR-04

Source: https://xmlvalid.com/rules/UBL-SR-04

> Invoice object identifier shall occur maximum once

- Level: error
- Rule sets: EN 16931 rules (UBL) 1.3.17, EN 16931 rules 1.3.16 (UBL), Romanian RO e-Factura rules (UBL) 1.0.9
- Context: `/ubl:Invoice | /cn:CreditNote`
- Test: `(count(cac:AdditionalDocumentReference[cbc:DocumentTypeCode='130']/cbc:ID) <= 1)`

Validate a document against these rules: https://xmlvalid.com/validate