I make a few mentions of Layer 9 in my Textured Backgrounds article, in the animations part, I believe.
I think Layer 9 was originally invented for water. As you can see in some of the beta screenshots, edges of the screen turn black, while in the final they just keep whatever was last there. Bleh, I'm not explaining this well, am I?
It can also be used for some weird eyecandy effects, but I'm not sure why you'd want to. Especially since most people think seeing Layer 9 at all is a bug. You should probably leave it alone.