# UBL-SR-52

Source: https://xmlvalid.com/rules/UBL-SR-52

> Document reference shall occur maximum once

- Level: error
- Rule sets: EN 16931 rules (UBL) 1.3.17, EN 16931 rules 1.3.16 (UBL)
- Context: `cac:InvoiceLine | cac:CreditNoteLine`
- Test: `count(cac:DocumentReference) <= 1`

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