midpipe is the one with the cats on it, and can (doesn't necessarily) include the J section. SLS makes a piece that both does and does not include this piece.
Mostly this is because there is an O2 bung on the J section that the proteges use, and the same bung is between the cats (about 3' futher back) in the middle of the pipe on the MSP. either will bolt up to any protege, its just a question of how much work will need to be done to rewire the secondary 02.