A "MAME 2003 Plus ROMset" is a curated collection of ROM files (the actual game data dumped from arcade PCBs) that have been verified to work specifically with this core.
MAME requires ROMs that match its internal database of checksums (CRC/SHA1). Because MAME 2003 Plus has a unique set of drivers (different from vanilla 0.78 and different from modern MAME), it requires its own dedicated . mame 2003plus romset
