# UBL-SR-39

> Project reference shall occur maximum once.

Checked against Peppol BIS Billing 3.0 2026.5

Canonical: https://einvoicecheck.eu/rules/ubl-sr-39

## Official rule text

Project reference shall occur maximum once.

## Why did my invoice fail UBL-SR-39?

The Project reference (BT-11) identifies the project the invoice relates to. EN 16931 allows at most one project reference per invoice, so cac:ProjectReference/cbc:ID must not appear more than once at document level. Use additional document or contract references for other linkage rather than repeating the project reference.

## Common causes

- Two cac:ProjectReference blocks at document level.
- cbc:ID repeated inside a single cac:ProjectReference.
- A multi-project invoice modelled with several project references instead of separate invoices.

## How do I fix it?

```
Keep one project reference: <cac:ProjectReference><cbc:ID>PRJ-2026-014</cbc:ID></cac:ProjectReference>.
```