Storing custom fields

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, 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.

Yes, according to the informal standard.
The question becomes: "Does whatever application you're using read customized ID3 tags?"

It has to be added that MP3tag does not show all present fields. The PRIV fields are always excluded and some others like TKWD as well. These do no show and you get no hint in MP3tag that these fields are there.