Performing either projection-based embedding or sDFT calculation offers the advantage that the respective densities can be associated with the two (or more) subsystems and that the total density of the overall system is a sum of the subsystem densities. By construction, then also the occupied molecular orbitals of the subsystem of interest I are mostly localized in the respective spatial region. It is therefore possible, to limit the GW-BSE calculation to this "active" region only. For this, no changes in the formulation of the actual GW -BSE steps is required as the embedding potential is simply another external potential already included in the preceding DFT-in-DFT calculation. This implies, however, that all quantities involved use the embedded Kohn-Sham molecular orbitals and their energies as starting point. As these are changed with respect to the full reference case, one should not in general expect to obtain the same excitation energies from a GW -BSE calculation after subsystem-DFT as from a full GW -BSE calculation.