Mr. Mikkel Heppe Brun, VP of Global Strategy for Tradeshift, has created a three-part review of the draft EU directive on e-invoicing: http://ec.europa.eu/internal_market/publicprocurement/e-procurement/e-invoicing/index_en.htm
These make for interesting reading:
Note where Mikkel makes reference to existing standards:
"Existing standards for e-invoices already published by CEN BII are used. The directive must mandate the ongoing standards effort already handled by CEN in BII."
... OASIS Universal Business Language (UBL) is an available choice and the most used of the choices available for a syntax expressing the semantics of a business document.