There are two type of ratings: auto rating, the one conjectured up somehow some where and My Rating, the one supplied by your self.

The blue ones are the ratings by WMP, the yellow ones by you.

You can use both type of ratings when creating an Auto Playlist.

Rate a composer

One day I was browsing my collection by composer.
Each composer has a rating. Adjust the rating and you can order the composers by rating. Nice.

Let’s give Beethoven the highest rating.

Guess what happens?

All the music composed by Beethoven get this rating because a song is a song is a song is a song.
Every time you edit a tag, you are editing songs and nothing else.