If you had Mega Man Zero 3 or Zero 4 inserted into Slot-2 of your Nintendo DS, you could access a bonus feature that let you battle four additional bosses from Mega Man ZX. All of the sprite and object data for these bosses is actually loaded by ZX from the GBA Game Pak and converted instantly to the more recent format.
This was probably done because the Japanese Mega Man ZX ROM is very close to 64 MiB in size due to having complete voiceovers. The data is loaded from the GBA Game Pak instead because including the Zero boss data would have pushed it past the limit and forced them to produce the game on more expensive 128 MiB Game Cards.
Because the sprite data for those bosses simply isn’t on the DS Game Card, it is also regrettably impossible to fight the bosses on a Nintendo DSi, even when using cheats to enter the Zero boss rooms. This patch fixes that by allowing the Zero boss data to be loaded directly from the Mega Man ZX ROM, doing away with the need to insert a GBA cartridge. You can now enter the Zero 3 and Zero 4 boss rooms without having to restart the game, which is a nice side effect.
