from the manual:

Calculating Characteristic Impedance
Each port in a structure being analyzed can be viewed as a cross-section of a transmission line.
HFSS computes the characteristic impedance of each port in three ways ÔÇö as Zpi, Zpv, and Zvi
impedances. You have the option of specifying which impedance will be used in the renormaliza-
tion calculations.
ÔÇó For TEM waves, the Zvi impedance converges on the port's actual impedance and should be
ÔÇó When modeling microstrips, it is sometimes more appropriate to use the Zpi impedance.
ÔÇó For slot-type structures (such as inline or coplanar waveguides), Zpv impedance is the most
HFSS will always calculate Zpi impedance, the im