In a preceding work, Observability Is Not Energy: Distinguishing Energy Redistributionfrom Detection, it was shown that energy conservation does not imply continuous observability,and that physical observation is limited by interaction channels and detector coupling. Thepresent paper carries that analysis forward by addressing a complementary question: why energysometimes persists in stable, localized configurations rather than redistributing freely. We arguethat mass may be consistently interpreted as energy whose redistribution is constrained, allowingpersistence without continuous radiation. This interpretation introduces no new physics andremains fully compatible with quantum field theory, general relativity, and thermodynamics.By making the role of constraint explicit, the paper clarifies why stable matter does not radiatecontinuously and why energy localization is a natural outcome of established laws rather thanan additional assumption.