i assume your referring to the type of exhausts for the celica.
the cat referrs to a catalytic converter it is part of the smog and emissions systems.
the axle referrs to the rear axxle on a car.
the exhausts for the celica are axle back as they do not replcae the resonator but mount where the rear axle would be if the celcia had one.
cat or cat back would be a complete exhaust sytem that woudl replace everything from teh cat back to the exhaust tip.
the only cat back sytem for the celica is teh kazumma and is illegal do to the soudn it produces is too loud for federal noise laws.
i hope that that was imformaitve enuf for you.