It was originally an expansion pack for Unreal2, because everybody was complaing that Unreal2 didn't have muliplayer. And This is the first public release of XMP for UT2004.
It's the same game, just that you can play it under UT2004 now. The beta does need some work as the AI doesn't seem to know how to play, but I am sure multiplayer would work fine (I have only played one SP match so far).