TARGET defaults to MAME. If you're doing multiple builds with different combinations of drivers it helps to use different SUBTARGET so they don't overwrite each other. It's also useful to give it a different SUBTARGET name to make it obvious that it isn't a full MAME build (even if the small size implies that). If you're setting SOURCES, you can make up any SUBTARGET and it will generate rules to build the executable with that name.