What is the main cat

[Daniel Murray]

I bought a clio 197 last week and it has a milltek decat, it is under the car (not under the manifold) I read on a forum that there is a main cat and secondary cat, is the one closest to the engine the main cat? if so, will I pass an MOT with the secondary cat decat. the exhaust is standard apart from this. will this exaggerate the "flat spot"?

Thanks.