Applying the reciprocity relation 

e) Now consider an enclosure consisting of N black surfaces maintained at specified temperatures. The net radiation heat transfer from any surface i of this enclosure is determined by adding up the net radiation heat transfers from surface i to each of the surfaces of the enclosure

f) A negative value for  indicates that net radiation heat transfer is to surface i (i.e., surface i gains radiation energy instead of losing). Also, the net heat transfer from a surface to itself is zero, regardless of the shape of the surface.