I am trying to understand better how tags work, and I think there are people here who can explain the following.
I understand that there are different systems of tagging (FLAC/Vorbis, id3, mp4). I normally use FLAC when I rip my CDs or purchase music via download. IIUC, FLAC files can include any tag that the user wishes, that is, they are not limited to a set of predefined tags. I use a couple of tags that I am sure are non-standard (Sub-genre, Group).
Recently I have started purchasing some DSD files (DSF format). Mp3tag can now work with DSF files, for which I am very grateful. But from what I can figure out, DSD files are tagged with id3 tags. I work with DSF files in Mp3tag exactly as I do with FLAC and my custom fields always seem to be there. Can files tagged under id3 contain customized tags? What about other formats like mp4/iTunes? I have looked at https://docs.mp3tag.de/mapping, which says that one can add custom fields with Options-->Tag Panel but does not explain exactly which formats can make use of custom tags; nor have I been able to find a clear answer through other internet searches.