That is exactly it. The memory card that comes with the N64 is a 4MB card that plugs into a slot in the front. The expansion pack is 8MB. Some games require the expansion pack in order to play. (Donkey Kong 64, Perfect Dark, Zelda: Majora's Mask to name a few). The 8MB option is the emulators way of emulating an expansion pack. Also, with the console some games may not require the expansion, but the expansion pack can enhance the games.