Many 90s and 2000s games (3D arcades) require chd files, which are large images of hard drives or CDs inside the arcade machine. These are separate from the main romset zip files. Where to Find MAME ROMs
The operational software required by specific arcade motherboards (like the Neo Geo MVS or Sega NAOMI) to boot the games. Understanding MAME ROM Set Types all mame roms pack