Formats / Guides
What is a CIUS? (Core Invoice Usage Specification)
A CIUS — Core Invoice Usage Specification — is a specification that tailors the European e-invoicing standard EN 16931 to the needs of a particular country, network, or industry by restricting it: making optional elements mandatory, limiting codelists, or adding extra business rules. A CIUS may never contradict the core standard, so every invoice that conforms to a CIUS also conforms to EN 16931 itself. Peppol BIS Billing 3.0 and Germany's XRechnung are the best-known examples.
What is a CIUS?
A CIUS (Core Invoice Usage Specification) is a rulebook that takes the EN 16931 core invoice model and restricts it for a specific network, country, or sector: it can make optional fields mandatory, forbid fields, narrow codelists, and add business rules — but it may never contradict or relax the core standard. A compliant CIUS invoice is therefore always a compliant EN 16931 invoice.
What can a CIUS change — and what can't it?
| Allowed in a CIUS | Not allowed |
|---|---|
| Make an optional element mandatory | Make a mandatory element optional |
| Forbid or ignore an optional element | Add information the core model cannot carry (that's an extension) |
| Restrict a codelist to fewer values | Extend a codelist with new values |
| Add stricter business rules | Relax or remove core BR-* rules |
Is Peppol BIS 3.0 a CIUS?
Yes. Peppol BIS Billing 3.0 is the most widely used CIUS of EN 16931: it restricts the core model with Peppol-specific rules (PEPPOL-EN16931-R*) and binds it to the UBL syntax for exchange over the Peppol network. XRechnung is Germany's national CIUS in the same sense. In practice that is why a Peppol invoice must pass both the core rules (BR-01, BR-CO-10, …) and the Peppol rule family — see Peppol BIS 3.0 vs EN 16931.
What is the difference between a CIUS and an extension?
A CIUS only restricts EN 16931, so its invoices stay valid against the core. An extension adds information the core model cannot carry, so an extended invoice is not guaranteed to validate against the core standard. XRechnung, for example, exists as a CIUS with a separate extension.
Related: UBL vs CII · supported formats · validate an invoice free.