ui cabinet = show the mewui (default) frontend on mame
ui simple = show the previous frontend on mame.
I know the option, name and choices make no sense at all.... and that mame's documentation, what there is of it, is severely lacking, and that's being nice about it..
The problem is that there's over 300 options in mame.ini (which is madness in itself), new ones get added all the time, and nobody has the time to say what they do.