Use of AlbumArtist

[PSH]

NaimUniti is the device.
I think I have it sorted but I don't quite understand some of the guidance on the website.
I added AlbumArtist to the indexTags (no other changes). I also added AlbumArtist to Album.displayFormat = {$album$artist$albumartist}.
This all seems to work and indeed there is clear software intelligence when artist and album artist are the same - they are not duplicated which is good.
However, I don't quite understand how the statement that "The Artist index provides main artist information. It contains AlbumArtist tag values for albums that have this tag and Artist tag values otherwise" quite fits with what is apparently going on.