Split sets separate the parent game from its clones. The clone zip file only contains the unique data altered for that specific version, relying on the parent zip file to be present in the same directory to boot.
Where this fits in the MAME ecosystem
This comprehensive guide covers everything you need to know about this specific ROM set, why it matters, and how to use it. What is MAME 0.139u1? Mame 0.139u1 Roms Archive
For users and archivists, these driver changes meant that the raiden2.zip or megadplk.zip files of this era are distinct from those in later versions, reinforcing the need for version-specific archives. Split sets separate the parent game from its clones
ROM set version must match emulator version closely (±0.001 minor releases). why it matters