mp4 tags vs id3 for m4a - conversion recommended or not?

I did some reading on aac & m4a tagging standards (a.o. on AtomicParsley), and differences with id3v2 (I am not really concerned with legacy id3v1), I have read of course the MP3Tag help pages on tags, and have some questions regarding this.

What I understood (in layman’s words):

  • mp3 files recommended tag format is id3v2 (metadata container), which is widely recognized/used; key points are a max 256MB of text in each tag
  • aac files (raw, i.e. not embedded in mp4 container) also support id3v2
  • m3a files (with embedded aac) have no standardized tagging system. iTunes/apple use their own system (called mp4?), with a max of 255 bytes per tag (except lyrics)

Now the questions …

  1. I read that m3a also can have id3v2 tags attached, and that iTunes also reads and supports this (although I read elsewhere that it is not supported). So is this possible? Do you need to use either/or, or can you have both at the same time?
  2. If id3v2 for m4a is possible, would you recommend to convert all these mp4 tags to id3v2 (and how to do this?)
  3. For my files that are listed in the MP3Tag navigation window as having mp4(mp4) tag, I assume that they use the apple/itunes tagging system (so with the max 255 bytes per tag?) So what happens if with MP3Tag I add an extended tag, which is not supported by mp4/apple/itunes (e.g. ORIGALBUM or ORIGARTIST), or if I put more text then 255 characters?

So, I did some testing myself :

On a m4a file (that has a MP4 Tag structure), I can add ORIGALBUM and ORIGARTIST as fields, and these are saved by MP3Tag, although these are not supported by the MP4 standard. Even when opening the file with iTunes, changing songtitle, saving again, and re-opening with MP3Tag, these two fields are still present (but the Tag structure was not changed to ID3v2, that supports these fields, but is still MP4).

However, when trying to make a field longer than 256 characters, MP3Tag truncates after 256 (actually after 253 as the last three characters are changed into ...). So in this case, the MP4 standard is followed.

Probably it is my limited knowledge how tagging works and is embedded in a file, but I am a bit lost here...
Could someone give some insights on what is going on here?
Thanks !