The ground plane is a virtual surface which simulates the surface on which the model is standing and onto which shadows are cast. The height (z-value) of this can be set by the user. The option to display shadows on this plane in addition to on the building, can be switched OFF, as can the display of zones below this ground level. The option Show DXF on ground plane will show an overlay of an attached DXF file on the ground plane (see ModelIT for details on attaching DXF files).
For the majority of circumstances, you will not have to change this from the default value of 0.0.
