Just to clarify, the CAT is the catalyst unit and it is located on the downpipe app. 50cm after the turbo. The reason for the part you want is called a catback is that it is located after the Downpipe which has the cat fitted to it, so the part you want is "Behind the cat", get it??
The box just in front of the rear bumper is an resonance box, some exhaust has none, some has one and others have two of them, basically the more boxes the quiter the exhaust system is. So to answer your originally question, no it is not the cat
My own APR RSC 3" system:
You can see 2 boxes, the one closest to the front of the car is the RSC box and the rear a normal silencer box.
This box you are asking about is NOT the CAT