The last metadata format in this comparison series will be metadata for AAC audio files, which are usually included in an MP4 container (see MPEG-4 Part 14). MP4 audio files can have various file extensions, most commonly m4a, but also mp4 or m4b (for audio books).
So let the confusion continue ...

MP4, AAC and iTunes-style Metadata

The MPEG-4 container format is based on Apple's QuickTime container and each MPEG-4 file must have a major file brand. For example, an AAC audio file typically lists M4A as its major file brand. See AtomicParsley for a description of the ...