Isolated event four-momentum balance

For the relativistic-particle-mechanics collision configurations of Finite event collisions and observer quantities, adopt the isolated-collision balance already stated in clause (iii) of Adopted massive-particle momentum, observer measurement and isolated-collision conservation: when the event is isolated and the incoming and outgoing lists contain every leg, $Q_{\mathrm{i}\mathrm{n}}=Q_{\mathrm{o}\mathrm{u}\mathrm{t}}$. This local postulate records the exact collision interface used on this page; it is a scoped reuse of the existing adopted model assumption, not a second independent conservation assumption.

The balance is an adopted physical premise. It does not show that any specified reaction occurs and supplies no interaction mechanism, selection rule, probability, cross section, or amplitude. The cited treatments motivate and explain the model formulation; they do not turn the postulate into a deductive theorem or a report of an observed collision.