What is recognised as an 'album'?

[simoncn]

Or alternatively, if there is no AlbumArtist tag, the Artist tag value(s) must match exactly.

This is to prevent albums like 'Greatest Hits' or 'Beethoven Symphony No. 9' by many different artists being merged together into a single album.

It is also necessary to have the mergeFolderAlbums property set to true (see this section).