The final determination of the exitance on each subsurface requires knowing the fraction of direct flux that the subsurface can radiatively exchange with all other subsurfaces. Since the surfaces are assumed to be perfectly diffuse, these surface exchange factors are purely geometric and are called Form Factors. They are determined in a two-step process: the unoccluded form factor for a pair of subsurfaces is determined and then modified by the occlusion factor for the pair.
Unlike configuration factors, there is no single, simple equation that can be used to calculate form factors. A numerical double contour integration process is used. See the technical article: “Calculation of Occluded Radiative Exchange Form Factors,” DiLaura, D.L., LEUKOS, July 2006, Vol 3, No. 1, pp. 51-67.