Also called Core Invoice Usage Specification
A Core Invoice Usage Specification is a national or sectoral narrowing of EN 16931. It may make optional fields mandatory and forbid values the standard allows, but it may never add fields the standard does not define. XRechnung and PEPPOL BIS are the two you will meet.
The rule that makes a CIUS predictable: it can only restrict. It may require something EN 16931 left optional, or forbid one of several permitted values, but it cannot introduce a field the European standard does not define.
That is why an invoice built for a CIUS is always valid EN 16931, while the reverse does not hold. Build to the strictest CIUS your customers require and you satisfy everyone below it.
It means "valid" is an incomplete answer unless you say valid against what. A tool that checks only EN 16931 and reports a pass is telling the truth about a question the buyer did not ask.
This is why our report carries an explicit incomplete state rather than only pass and fail. If your invoice declares a CIUS we have not checked, it says so. Getting that distinction wrong is how a validator hands someone false confidence, which on a compliance document is worse than handing them nothing.
Read out of BT-24. If it names a CIUS we have not run, the verdict is incomplete rather than a pass.
