Regional variants or updates (e.g., the Japanese version). These require the Parent ROM to be in the same folder to work.
According to the leak, this wasn't just a repack of the usual MAME roms. This was a curated, perfect set: sega model 3 rom archive exclusive
. While there is no singular "official" archive, the community relies on curated "ROM sets" to ensure compatibility with the primary emulator, Supermodel Essential ROM Archive Details Regional variants or updates (e