Measuring S-Parameters,plz help

From FIG.3 said:"In order to receord S11 and S22 we have to ensure that on the output side the line impedence Z0 is matched for a2=0 to be enforced ..." I dont understand Why ZL=Z0 can get a2=0 ? I think If the output impedence of [S] macthed with Z0 that can get a2=0 ,I know I'm wrong ,but plz tell me why.
Thanks!

Here we have a loaded line connected to [S]. To determine S11 we need to make sure that a2 = 0. This is achieved when the loaded line is matched to the output of [S]. But I think that it is assumed that the output impedance of [S] is the same as that of the line, that is, Z0. Then we require that ZL = Z0. And please understand that not just the line needs to be matched, but the loaded line. Otherwise an outgoing wave would be reflected at the load.

but here its all together a different story.... i mean the question asked is not yet clear... and i didn't get the wordings in this figure below!

Where should the inpendance matching be done and why?