What's the difference between fast sweep and interpolating?

Dear Sir:

I wonder doing a simulation on waveguide. But as the tutorial, it uses the interpolating for sweep type. What's the difference between fast sweep and interpolating?
And I heard in certain case, we need change the 'edit impedance Mult' of exitations from '1' to '0.5'. Why should we do that, and it is used in what kinds of cases?

Thank you very much!

Regards,

Wei

for the frequency sweeps check the pdf file called hfss_onlinehelp in hfss9 help directory
page 405

for the multiplying factor u should use it if u r using symmetry planes check page 417 in the same file