When you have lots of images stored next to your media, we take note of the file names.
So, if you have back.jpg, we’ll show that right after the front cover, and you can find the album using the “Has Back Cover” focus.
I don’t remember the whole spec, but we do something similar with images like booklet2.jpg, booklet3.jpg, etc.
Point being, there is a difference between “Has Liner Notes” and “Has PDF”.