How does the lumped port (HFSS) or discrete port (CST) work?

Hallo,

in fullwave simulation, waveguide ports are used to excite the structure with energy. I am talking about driven problems only.
A 2D Eigenmode solver determines the excitation field in preprocessing, and considers it as time varying boundary condition in the solution process.
The characteristic impendance and gamma are also known from the preprocessing. This enables us to deembed afterwards.

All clear. But how does the lumped port (HFSS) or discrete port (CST) work.
Fixed reference ? Why ?
Can somebody please give me informations about lumped ports.

Thanks