# CIUS

Country or sector specific usage specification that constrains EN 16931.

## Definition

Country or sector specific usage specification that constrains EN 16931.

## Why this term matters

A CIUS is what turns the abstract EN 16931 model into something a country, sector, or platform can actually validate against — XRechnung, Peppol BIS Billing 3.0, NLCIUS, and OIOUBL are all CIUSes. Misidentifying which CIUS applies (or sending an invoice with the wrong CustomizationID URN) is the most common cause of cross-border interop failure.

## Explanation

CIUS rules typically tighten cardinalities, enforce identifiers, or restrict code lists.

XRechnung is a well-known CIUS for Germany (B2G).

## Common mistakes

- Sending an EN 16931 generic CustomizationID (urn:cen.eu:en16931:2017) when the recipient expects a CIUS-specific URN (e.g., #compliant#urn:fdc:peppol.eu:2017:poacc:billing:3.0).
- Layering two CIUS rule sets that contradict (e.g., NLCIUS + Peppol BIS) without checking that the document satisfies both.
- Validating only with EN 16931 Schematron — CIUS-specific rules (BR-DE, BR-NL, BR-IT) are additional and not part of the base ruleset.
- Treating an Extension and a CIUS as interchangeable — Extensions add new BTs while CIUSes restrict existing ones.

## Frequently asked questions

### Can a single invoice satisfy multiple CIUSes?
Yes — a UBL or CII document can declare a CustomizationID for the strictest applicable CIUS and still be EN 16931-conformant. For example, an XRechnung that follows BR-DE rules is automatically EN 16931-conformant. Stacking two unrelated CIUSes (e.g., NLCIUS and XRechnung) is rare but technically possible if the recipient publishes both.

### How do I know which CIUS a recipient requires?
In Peppol, the recipient publishes accepted document types and customization IDs in their SMP entry. Outside Peppol, the recipient typically documents the requirement on a supplier-onboarding portal or in tender documents.

### Where is the CustomizationID located in the XML?
In UBL it is cbc:CustomizationID at document root. In CII it is rsm:ExchangedDocumentContext/ram:GuidelineSpecifiedDocumentContextParameter/ram:ID. Both are top-level and one of the first elements a recipient parses.

## Related resources

- [XRechnung guide](/resources/compliance/xrechnung-standard)

## Related terms

- [EN 16931](/resources/glossary/en-16931): European standard defining the semantic data model for e-invoices.
- [XRechnung](/resources/glossary/xrechnung): German e-invoicing specification for the public sector based on EN 16931.
- [KoSIT](/resources/glossary/kosit): German coordination body; commonly refers to the BR-DE validation rules/validator for XRechnung.
- [Validation](/resources/glossary/validation): Process of checking an e-invoice against syntax rules and business rules.