Artist Variety Rate is the number of different artists as a percentage of the total number of albums. The Media and Country listings also serve as a key to the abbreviations used in the record list. The term Album refers to all media types which now include artist-related Books and Videos. By Albums chart.
Genre refers to a major musical style while Category can be considered a 'sub-genre' of that style.
It's still crude and inconsistent. I haven't decided on the level of detail and general scheming yet.