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UBL vs CII
EN 16931 defines the meaning of an invoice, not a single file extension. Europe serialises that model as UBL 2.1 or as UN/CEFACT CII.
Peppol BIS Billing 3.0 is UBL. XRechnung may be either syntax. ZUGFeRD and Factur-X embed CII inside a PDF/A-3.
Pick the syntax the receiver named. If you must convert, do it through a profile-aware converter so CustomizationID, business rules and tax categories stay intact.
Related
Questions
Is converting UBL to CII lossless?
For a standard EN 16931 invoice, yes in practice. Extensions outside the core model, or a CIUS the other syntax does not allow, will be rejected rather than silently dropped.