They have sockets that are compatible with the omni part arms sold separately. The Prime versions have male pegs on the arms and female sockets on the torso. The omni parts are mostly female.
I believe the Mad Cat torso has the sockets filled in that usually hold the torso weapons too. Not positive on that one though. It also has some boxes where the LRMs would sit on top. This is for variants that use different shoulder missile boxes.
Dak
