:(
I know the parts come with all sorts of names like rear slam panel mainly used for the front of the car but have heard it called that. I did think it was welded onto the cars at the rear on Subarus but without fully looking I'm not 100% :( but I was only under the impression it was that then the foam part then bumper skin, I.e the rear bumper. if it is the part you must of hit it dam hard to damage it.
I've not checked the guides yet myself it's my geek night so have been a bit busy, will have another look tomorrow.