Currently, the PDF rendering extension supports three versions of the standard – PDF/A-1b (2005), PDF/A-2b (2011), PDF/A-3b (2013).

I am generating invoice that need to be compliant with the Factur-X standard (similar to ZUGFeRD in Germany)

It is a PDF/A-3u compliant file where the human visible PDF is still there as a main document (like before), but where there is an XML File attached/included, which is standardized.