M4A Mode Descriptions Requested

When dealing with M4A files, the uses I usually see are Mode 1 & Mode 2.
How do I interpret these?

I know M4A can handle up to 48 channels: is this related?
I mean, does Mode 1 = mono & Mode 2 = stereo & so on up the scale?

I've googled til I'm blue in the face but I can't find a definitive answer.

The mode column for MP3 of course is mono, stereo, joint stereo, etc. but M4A is numerical, so what I'm looking for is a definition of the possible values.

Also: Does M4A have MP3 equivalents for both stereo and joint stereo?

Thanks in advance