Well, I can only say that I’d like to see a detailed explanation of what is, and isn’t supported.
I find it strange that the Discnumber tag is not definitely supported in Ogg and FLAC files, surely the way it works is initially the file format is checked and one finding what format is being read the relevant code is called. Then the tags are found, read out into a standard format buffer and processed according to what is supported. That way you only need one routine that parses tags, and a small separate routine that finds the tags for each format.
At least, that’s what I expect to have.
If there is a detailed guide then I have yet to find it.
