Not exactly.
The NDS normally allows you to play DS games from slot1 and GBA games from slot2. There are two ways to play both DS and GBA games: slot1 carts and slot2 carts. Both slot1 and slot2 carts can store/play both types of games, but take different approaches to allow this.
The DSTT is slot1 (as it goes in slot1 of your NDS). Since normally DS games only play from slot1, slot2 carts require a "passme" cart in slot1 to play .nds games from slot2 (the passme "tricks" the DSTT to view the slot2 device as a slot1 device so the DS games can load). The DSTT can act like a passme if you happen to have a slot2 cart that you want to run games from.
Note that the GBA expansion packs are different than slot2 carts (even though they also go in slot2). Since GBA games can normally only be played from slot2, a GBA expansion pack is required to allow you to play GBA games from your slot1 cart. GBA expansion packs only need enough internal memory to hold a single GBA game. The GBA games stored on the microSD in your slot1 cart and are transferred to the GBA expansion pack to play GBA games. Most expansion packs can also use their internal memory as a cache required to run the Opera browser.
Most recent carts are slot1 carts because they don't require that you have anything in slot2 to play DS games. If you have a DSTT and don't own a slot2 cart, you won't use the passme mode for anything.
Hope that helps.
-Bri