What kind of metadata can be stored in m4a files?

What metadata can an m4a file hold and how do I get at it?

Metadata for .m4a apple lossless (ALAC) files

My new music library is all nicely ripped Apple Lossless files in .m4a containers. These contain metadata, but it’s not id3v2 and the universe of mp3 tools mostly don’t work. ffmpeg’s ffprobe sort of works but doesn’t show all tags.

What does work for displaying m4a metadta is mp4v2, installable via Homebrew. Docs here. It seems pretty stable but still gets some updates.

(imac) = mp4file --list test.m4a
BRAND  COMPAT              SIZING  FILE
----------------------------------------------------------------------
M4A    M4A,isom,mp42       32-bit  test.m4a
 
(imac) = mp4info test.m4a
mp4info version 1.9.1
test.m4a:
Track   Type    Info
1   audio   alac, 260.806 secs, 562 kbps, 44100 Hz
 Name: Pastourelle (E passo de dossai: Ah! viens pres de moi), folksong for voice & orchestra (Chants d'Auvergne, Series 2, No. 1)
 Artist: Dawn Upshaw
 Composer: Joseph Canteloube
 Release Date: 1994
 Album: Canteloube: Songs of the Auvergne
 Track: 1 of 15
 Disk: 1 of 1
 Genre: Vocal Music
 BPM: 0
 Part of Compilation: no
 Cover Art pieces: 1
 Album Artist: Kent Nagano
 Media Type: Movie
 TV Episode: 0
 TV Season: 0

Source Metadata for .m4a apple lossless (ALAC) files