Thanks Ivan and Martin,
Ivan, yes, there is a standard somewhere. Haven't read it all yet but it is mentioned something in the "little" pdf document I linked to in my first post. Seems to be some EU standard with some minor changes to "fit" local laws.
I have to read more to get the full picture.
Martin, this is a system for sending orders and invoices ,and more, between companies and also the authorities as they prefer this format to ease document flow when purchasing. They import the invoices to their accounting systems. Less manual work for the receiver and faster handling of information.