How is support pressure defined in Unwedge?

Support pressure is discussed in the Design Factor of Safety section of the Unwedge Online Help.

Basically, it is the amount of pressure applied to the face of the wedge (on the excavation) to achieve the design factor of safety.  In other words, if you add an active internal pressure to the excavation, normal to the excavation surface, with a magnitude equal to the support pressure, the factor of safety for the wedge would be the design factor of safety.