The Define Plane Location dialog allows the user to digitize the location of a plane for the Locked Plane digitizing mode or for the Project To Plane filter.
This dialog allows a point in space to be chosen as the origin of the
plane 
the plane will pass through this point. Given the
point digitized and the plane, the offset distance from the plane will
be calculated and updated.
This operation must occur after selecting the desired plane. Subsequently selecting a different plane will change the origin position.