Recent progress in modeling light-matter interaction in terms of Resonant/QNM interactions is reviewed. It is argued that such descriptions can be refined by formulating them in terms of scattering response functions like S and T -matrices. This methodology allows the derivation of sum rules, which in turn can be used to formulate approximations that satisfy underlying conservation laws. This discussion is demonstrated in the concrete case of a simple, solvable, scattering system, with applications to more complex geometries will also be discussed.