Mega Man ZX had a bonus feature where if you had Mega Man Zero 3 or Zero 4 inserted into Slot-2 of your Nintendo DS, you could fight four extra bosses from that game. ZX actually loads all of the sprite and object data for these bosses from the GBA Game Pak, and converts it to the newer format on-the-fly.

This was presumably done because the Japanese ROM for Mega Man ZX, due to having full voiceovers, is very close to being 64 MiB. Adding in the Zero boss data would have pushed it over the limit, meaning they would have had to produce the game on more expensive 128 MiB Game Cards; so instead, the data is loaded from the GBA Game Pak instead.

Unfortunately, this also makes it impossible to fight the bosses on a Nintendo DSi, even when using cheats to get into the Zero boss rooms, because the sprite data for those bosses just isn't on the DS Game Card. That's where this patch comes in: it makes it so that the Zero boss data is now loaded from the Mega Man ZX ROM itself, eliminating the need for a GBA cartridge to be inserted. A nice side effect is that you can now also enter both the Zero 3 and Zero 4 boss rooms without needing to restart the game.