For what it's worth, AlbumArtist() and [Album Artist (auto)] are exactly the same thing.
Anything in brackets actually becomes Field(...). So [Album Artist (auto)] is the same as Field(Album Artist /(auto/)).
And if you look at the field 'Album Artist (auto)' in Options > Library & Folders > Manage Library Fields..., you'll see it's the expression AlbumArtist().