Some sites sell CD/DVD collections of ROMs bundled with an older MAME version and claim you need a “registration key” to activate “extra” features. This is a scam. The key either:
In the late 1990s and early 2000s, some third-party compiles of MAME added registration dialogs as a way to solicit donations or to lock features. These were never official. The “extra key” might have unlocked: extra mame registration key
To help you find the right path for your arcade setup, let me know: Some sites sell CD/DVD collections of ROMs bundled
Modern versions of official MAME come with an improved built-in graphical user interface that is completely free. extra mame registration key