As far as I understand there are multiple XML invoice formats and EN 16931 accepts at least two: UBL and CII. At least in theory. I have no idea how it is going

As far as I understand there are multiple XML invoice formats and EN 16931 accepts at least two: UBL and CII. At least in theory. I have no idea how it is going to work out in practice, but I will learn the hard way :-) I have invoicing software as side-project and I have decided to make it usable in EU.