If, for all tracks in an album, Album Artist = Blank AND Album is the same for all of them AND they're all in the same directory, then Album Artist (auto) = Artist.
If, for all tracks in an album, Album Artist = Blank AND Album is not the same for all of them AND/OR they are not all in the same directory, then Album Artist (auto) = "Multiple Artists"
If, for all tracks in an album, Album Artist = "XXXX", then Album Artist (auto) = "XXXX"
Basically, Album Artist (auto) is an easy way of letting MC decide which albums are Multiple Artist albums and which ones aren't. You set the Artist field for each track and MC sets the Album Artist (auto) value for the album. You can override Album Artist (auto) by setting Album Artist on the tracks (for example, you might want a techno mix to have Album Artist (auto)= the DJ who mixed it, not Multiple Artists so you set Album Artist to the DJ's name on the tracks). The idea is that you can always use Album Artist (auto) without worrying whether you've remembered to set Album Artist for each individual track.