Electronic invoices that are compliant with:

• the latest version of the European standard on electronic invoicing, which describes the semantic data model for electronic invoices;
  
• one of the syntaxes appearing in the latest list published by the European Commission.

Currently, the applicable European standard is EN 16931-1:2017, and one of the following 2 syntaxes must be used:

• UBL (Universal Business Language) XML, as defined by ISO/IEC 19845:2015, and maintained by the non-profit organisation OASIS Open;
• UN/CEFACT CII (Cross Industry Invoice) XML, developed by UN/CEFACT based on the XML 16B (SCRDM — CII) schemas.

Further information on the European standard and the syntaxes can be found here.

A copy of the European standard can be downloaded here.  

Information on the rules that must be followed to produce electronic invoices that are compliant with the standard, as well as sample XML files, can be found here.