Source IGN:
There will be three different types of cards created for Super Mario Advance 4: Demo cards, Item cards, and Level cards. Demo cards are simply encoded with animation data that will play out specific scenarios automatically, showing gamers how to pull off particular tricks and moves. Item cards give players the opportunity to scan in useable power-ups whenever they need it; looking for a frog suit, music box, fire flower, or even a power mushroom? Find the card in a set and scan it in. The game's already a super challenge, and these power-up elements will definitely help players along the way.
But it's the Level cards that are the most significant of the bunch. On these Level cards will be an entirely unique level created specifically for that card. The data strip holds all the level information, and when uploaded to the game it pieces together that level using the existing elements within Super Mario Bros. 3, from platforms to question blocks to enemy locations. See, Nintendo can conceivably create an infinite amount of challenges through several different series of cards, either bought in stores or included for free in places like the company's own Nintendo Power magazine. The level card that's included in the box barely scratches the surface of what Nintendo plans with the feature, since these level cards can also activate gameplay elements not found in the actual Super Mario Bros. 3 game. Remember the turnip-throwing power of Super Mario Bros. 2 or the Cape power-up of Super Mario World? e-Reader levels can and will use these elements, plus new ones: in some levels, Mario can even wield the power of a boomerang used exclusively by one of the enemies in the main game. Nintendo will reportedly even create cards featuring levels from the original Super Mario Bros. game.
The Game Boy Advance cartridge even features an insane amount of storage space. Not only will it save as many as 32 scanned-in levels from the e-Reader and store three different player slots for the main Super Mario Bros. 3 adventure, it can also record two different player movies. These movies are simply controller and button recordings from a specific level, and players can save a particularly cool performance in a level to cartridge for future viewing. Nintendo poured an awful lot into what may be the last of the Super Mario Bros. remakes on the GBA.