What is the significance of zero pressure on the boundary in Unwedge?

Zero pressure on the boundary means that the boundary is free draining and there is no pore pressure allowed to develop on the tunnel surface.  If this is off, it is as if drainage is prevented due to blockage of the drainage system and pore pressure is allowed to build up to the hydrostatic condition at all points on the joint surface.  It's the worst case situation and you'll find that, in general, it gives a lower factor of safety.