The difference between "pec" and "pec+infinit

The selection only affects the calculation of near- and far-field radiation during post processing. The 3D Post Processor
models the boundary as a finite portion of an infinite, perfectly conducting plane.


The words above is from the help of HFSS.But I am very clear about it.

The two 3D radiation pattern is obtained by simulating an circular patch antenna. All the definations are the same except for the groud plane.
Could anyone give some explanation in detail to it?