The emulator code is open source. However, ROMs are copyrighted . The MAME 2003 Plus archive contains data dumped from arcade cabinets. Legally, you should only download ROMs for games you physically own the original arcade PCB for.
Overview
These ROM sets are not random; they must match the of MAME 2003 Plus to function correctly. Mame 2003 Plus Roms Archive
: Over 95% of original MAME 0.78 ROMs are compatible, but the "Plus" version adds support for hundreds of additional games and backported bugfixes from later MAME releases. The emulator code is open source
is a community-driven fork of that classic codebase. The "Plus" signifies hundreds of backported fixes, new driver additions, and gameplay improvements from later versions of MAME (up to 0.188). It offers the best of both worlds: the lightweight speed of early 2000s MAME with the compatibility of modern emulators. Legally, you should only download ROMs for games
codebase (originally Xmame 0.78). Because of this lineage, approximately 95% of standard MAME 0.78 ROM sets are compatible with the core without modification. However, the "Plus" designation refers to several hundred additional games and thousands of bugfixes that have been backported from later versions of MAME. The Reference ROM Set