# UBL-SR-04

> Invoice object identifier shall occur maximum once

Checked against Peppol BIS Billing 3.0 2026.5

Canonical: https://einvoicecheck.eu/rules/ubl-sr-04

## Official rule text

Invoice object identifier shall occur maximum once

## Why did my invoice fail UBL-SR-04?

The Invoice object identifier (BT-18), an identifier for a referenced object such as a business object the invoice relates to, may occur at most once. In UBL it is carried in cac:AdditionalDocumentReference where cbc:DocumentTypeCode equals '130', and only one such reference of that type is permitted.

## Common causes

- Multiple cac:AdditionalDocumentReference blocks are given DocumentTypeCode '130' when only one is the object identifier.
- Object reference is duplicated alongside an attachment reference sharing the same type code.
- Loop over related objects emits several object-identifier references.

## How do I fix it?

```
Provide exactly one: <cac:AdditionalDocumentReference><cbc:ID schemeID="...">OBJ-123</cbc:ID><cbc:DocumentTypeCode>130</cbc:DocumentTypeCode></cac:AdditionalDocumentReference> and change or remove other references using code 130.
```