# UBL-SR-39

Quoted from a public specification. This page is not a validator, and it is not tax, legal, or accounting advice.

- Country: EU
- Syntax: UBL
- Severity: fatal
- Statement language: en
- Last verified: 2026-10-18

## Official statement

> Project reference shall occur maximum once.

Source: https://docs.peppol.eu/poacc/billing/3.0/rules/ubl-tc434/UBL-SR-39/

Peppol BIS Billing 3.0, May 2026 release, EN 16931 model bound to UBL

Severity note: fatal, from the Flag column of the Peppol BIS Billing 3.0 UBL rules index retrieved 2026-10-18.

## Plain-language reading

The project reference shall occur at most once. The statement ends with a period and says shall. It is 43 characters. The flag is fatal. The context is the same string as BR-CO-18. The published test counts cac:ProjectReference/cbc:ID. The count comparison has a space before <= and a space after it.

## What to change

Send at most one project reference identifier.

## Illustration

Illustration of the condition in the official statement. Not an example published by the source.

**Fails the stated condition.** The invoice has two project reference identifiers.

**Passes the stated condition.** The invoice has one project reference identifier, or none.

## Published context and test

Copied from the source. Not reimplemented on this page.

```
/ubl:Invoice | /cn:CreditNote
```

```
(count(cac:ProjectReference/cbc:ID) <= 1)
```

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