Why we need both input and output interception for IIP3 simulation in ADS?

Hi, I need to simulate IIP3 with ADS? I found there is a macro in ADS, named ip3_in(Vout,0,(1,0),(2,-1),50) and ip3_out(Vout,(1,0),(2,-1),50), can any one tell me why we need both input third order interception and output third order interception? What is the difference?
If there is a buffer following the LNA? Then how to specify the referrence impedence? Is it still 50ohms? Thanks very much...

IIP3 is more for RX's
OIP3 is for TX's / Amp's / PA's

Cheers

iip3 is input ip3 and oip3 is output ip3.
if you have iip3 and conversion gain you have oip3 and vice versa.