I think you are referring to the third cat, right? It's also known as the rear cat since the other two cats are connected right to the exhaust port on the engine block.
The diagram of the third cat in the service manual shows that it has 3 studs on either side (front and back) that have bolts connecting them to the pipe. The front of that cat leads to the J-pipe that goes up to the engine, the back of that cat connects to the pipe that goes to the resonator and the muffler. In most cases, these nuts that hold the two pipes mentioned are in poor shape and need to be cut off. The studs are welded to the cat so they don't move at all.