What is CII (Cross Industry Invoice)?
Short answer
CII (Cross Industry Invoice) is an international XML standard developed by the UN/CEFACT organization. Like UBL, it represents one of the official syntaxes supported by the EN 16931 standard — an electronic invoice according to EN 16931 can be written either in UBL or in CII, if it meets the requirements of the standard.
Not OASIS like UBL
CII is developed by an international organization other than UBL.
Another XML structure
CII writes the same Business Terms EN 16931 in a different syntax.
UBL is the standard of the Peppol network
Peppol BIS Billing 3.0 uses UBL, not CII.
CEN/TS 16931-3-4
The document describes the mapping from EDIFACT via EN 16931 to UBL or CII.
Conclusion
CII is a full-featured but not used syntactic implementation of EN 16931 in the Peppol BIS Billing network — Peppol requires UBL.
Not legal advice. Machine-translated answer, not yet manually reviewed. For the exact wording, see the Slovak original.