natively
a ds flash cart flashes the gba rom to the 3-in-1 to play,
you can also burn the image to the cart so you can play that 1 rom without having to reflash it, it will work on other devices by its self like a gba player, gba without needing a ds flashcart everytime, if you want to change the rom you need to reinsert it into a ds/dsl and burn a new rom onto it
or you could just pay the extra $10-15 and get a stand alone ez-flash iv