[Hectic]

A rubber and a bucking are the slave that go over the barrel.
The nub is either the little round bit (stock) or the bridge or flat tensioner or some home brew option or mnub.
The rhop pad is attached to the barrel, inside the hop window (filling it up and sticking out a little to the inside of the barrel). That gets used with a rubber/bucking and then does its job, the rubber itself is now mostly just for airseal. Then you would use a nub of some kind.
In summary
Rubber, bucking=the sever over the barrel
rhop=a silicone (or other) type of tubing cut to fit and then glued into the hop window
Nub, tensioner=the little bit that the hop up arm pushes into the bucking and then either the BB or the rhop pad