What is the space above a DS game in it's case for?

When you buy a DS game there is a area to put something in above the game, what is that space for?What is the space above a DS game in it's case for?
gameboy advance game cartidges in case you still play them. kind of lik ein the pokemon games when you need the old cartidges to catch specific pokemon in the DS gameWhat is the space above a DS game in it's case for?
That space above the ds game is an area for putting a game boy advance game in. Like for example: you have pokemon diamond and you are taking pokemon from firered int o your diamond and you want to keep the two game together.
its a GBA slot :)





can whack gameboy advance games in it...not sure why there still there tbh!

good makeup