I realised that my initial pot was probably very confusing.
My simple of question is this:
Ex: I have a compilation album, some files are FLAC and some are MP3. Each track has a different artist and album associated with it. What I would like to do is have some kind of album-artist field that says "My Mix, Summer 2004".
The problem is, as I understand, there is not a common field between ID3 and FLAC tags that I can use. For Id3 I will have to use TPE2/"Band" and for FLAC I will have to use "albumartist". That's no good, as my music player should want a common album-artist field across all the files, regardless of which codec they use. Otherwise, I cannot sort FLAC and MP3 files together.
So, I will have to create a user-defined tag in either my MP3 files or my FLAC files.
Am I right?
With albums that are not compilations, I will add an album-artist field anyway. That way, I can sort all my files by album-artist and easily find what i want.
Thanks in advance!