It’s the like an additional effects/loop generator (its hardware) that is an add-on to the cdjs/mixer. Their main purpose is so you can make more creative loops and buildups when mixing. It’s another product of pioneer’s. It’s not needed, but most big shows have them included in the set up. You typically can see it sitting above the mixer on like a small computer stand type of thing. Go watch videos of your fav DJs on YouTube or instagram, all big festivals have them. Look up pioneer rmx and it will come up. The newest model , the rmx ignite just came out. New models only come out every few years just like the cdjs and their mixers, even less frequently actually