I am pretty sure he is referring to the type of components inside your cat. Cat's are measured in CPSI which means cells per square inch, the lower the number the higher the flow (I think anyways), so first you want to make sure you get a high flow cat to get the exhaust gases out as quick as possible. Now the internals of the cat or substrate can be made from ceramic or metal with the metal ones more resistant to thermal and physical shock and can withstand extreme temperatures. Having said that plenty of high flow cats have ceramic substrates - have a look at justjap.com - ceramic high flow for $179 whereas metal substrates are closer to $400.
Sooooo, get a highflow cat, for a standard turbo running 11psi don't think you really need a metal substrate cat, save the money and use it to get yourself a 3inch dump/front pipe.
Now having said all this, I am running a stock exhuast on my R33 GTST and the above is what I've learnt as I research a 3" turbo back setup for my own car.
I still don't know whether an integrate dump/front pipe is better than separate components or if a bellmouth is better and lastly if a dual pipe front pipe is better than the single set up.
Hope some of the above helps.