Causality and space-time uniformity are the creative principles of source theory. Uniformity in space-time also has a complementary momentum-energy implication. It is illustrated by the extended source concept. Not only for that special balance of energy and momentum involved in the emission or absorption of a single particle is the source defined and meaningful. Given a sufficient excess of energy over momentum—an excess of mass—several particles can be emitted or absorbed. The description of the coupling between sources must include an account of such multi-particle exchange acts.