How to select the solution type???

In simutlation of hss, what diference is between driven modal and driven terminal , how to select them?

I look up the help of hfss, but I cann't understand what are the modal-based S-parameter and terminal-based S-parameter.
Could you give me the detailed explanation above problem?
Thanks a lot!!

As I understand, modal based S-pars normed to port impedance (for each frequency point). Terminal based S-pars normed to fixed impedance value (usually 50 Ohm).

Regards,
Kit

P.S. I'm also think, that the HFSS user's manual is very laconic and isn't very clear.