Scattering matrices of subshields and their norms have been used to relate the internal signals of an enclosed volume to an electromagnetic source environment. Both the line and aperture penetrations are included in the scattering matrix formulation. Experimental and analytical methods are proposed for the determination of the elements of the scattering matrices corresponding to these penetrations. These methods can, in turn, be used to analyze the overall shielding performance and to allocate the subshield electromagnetic hardening requirements of a system. Limitations and assumptions in using this electromagnetic topology technique are also addressed.