AAC and M4A formats?

My car radio supports AAC format audio files, but they must have the file extension "m4a" (apparently iTunes AAC format).
When I view files having "aac" extension with mp3tag I see that they have some tags, but if I rename the files to extension "m4a" mp3tag does not recognize the tags, and I get an error message if I try to save a tag to a file with "m4a" extension.

Do files with "aac" and "m4a" extensions have different formats?
Is there a reason why I cannot set a cover image in "aac" files?

Do a search with Google and use the following as your search term.

site:community.mp3tag.de aac

Most players will support M4a but not AAC (AAC is the raw format) and M4a is the container format (most commonly used by Apple in it's iTunes store)

Note that: Raw AAC only supports id3v1 and not ID3v2.