This document discusses Poynting's theorem and reciprocity relations for discontinuous electromagnetic fields across interfaces using a distributional approach. It examines three cases: 1) a dielectric-PEC interface with only electric surface current, 2) a dielectric-dielectric interface with no surface currents, and 3) an interface between two arbitrary media with both electric and magnetic surface currents. For each case, it applies Poynting's theorem and the reciprocity theorem to derive boundary conditions in terms of a surface impedance tensor Z. The document concludes that interfaces can be rigorously treated as parts of space using a distributional approach and described using surface impedance tensors rather than boundary conditions.