What is CIUS (Core Invoice Usage Specification)?
Short answer
CIUS (Core Invoice Usage Specification) is the mechanism by which EN 16931 allows individual countries or sectors to refine the use of the standard — for example, to tighten the cardinality of some data — without changing its data model. CIUS does not introduce a new XML format nor must it conflict with the EN 16931 standard.
Clarification, not change
CIUS tightens or supplements the use of EN 16931 for a specific country.
XML remains UBL/CII
CIUS does not change the syntactic format, only the data usage rules.
Peppol BIS transpositions
The official document is called something other than "SK CIUS".
German XRechnung
Different country, different specification, same basis EN 16931.
Conclusion
CIUS is a principle of national specification EN 16931; in Slovakia, it is implemented through the document Transposition of the Peppol BIS standard under the conditions of Slovak legislation, not through the separately named "SK CIUS".
Not legal advice. Machine-translated answer, not yet manually reviewed. For the exact wording, see the Slovak original.