I'm confused. I read a line in a book where there was a qualitative explanation about the optical theorem, namely that it is connected with conservation of particle number. But I think that this is only true if we just consider elastic scattering. Inelastic processes would of course allow a change in the particle number so there is no conservation. The fundamental connection of the optical theorem is that with conservation of probability (because it follows from the unitarity of the S-matrix). Is my "explanation" correct, or am I missing something?