How does the HFSS calculate the impedance of a lumped port?

For the waveport, the definition of the input impedance is clear. But how does HFSS calculate the input impedance of a lumped port? Does the port have to be referenced to ground? Want to know a little more in detail but can not find it. Hopefully someone can help me. Thanks!

HFSS does not calculate the port impedance of a lump port. It calculates the impedance only for a waveguide port. First determine the impedance of a line using a wave guide port then specify the impedance as the impedance of the lump port.

the port fields of the lump port is quite different from wave guide port, so the results are not really accurate.

Regards

Manu