This paper develops a structural interpretation of entropy grounded in spacetime geometry rather than microscopic state counting. The central proposal is that spacetime imposes a finite carrying capacity on admissible correlations across geometric interfaces, and that entropy measures the saturation of this capacity. We introduce the concept of Admissibility Carrying Capacity (ACC): a non-dynamical geometric bound on the number of independent relational degrees of freedom that can be simultaneously maintained across an interface such as a spatial cut, null surface, or causal horizon. Within this framework, entropy is redefined as the extent to which admissible relational structure exceeds interface capacity and must therefore be coarse-grained or rendered mutually inaccessible. This perspective yields a unified account of several well-known results without modifying general relativity or quantum mechanics. Area laws for entropy arise naturally from finite interface capacity; horizons are identified as saturated admissibility interfaces; and the Second Law follows as a capacity theorem rather than a statistical tendency. The framework also clarifies the status of several universal constants—most notably the Planck area and the Bekenstein–Hawking entropy normalization—which are interpreted as capacity constants rather than as outcomes of microscopic state enumeration. The work is intentionally conservative and structural in scope. It does not propose new dynamics, degrees of freedom, or microscopic models, nor does it attempt to derive interaction constants or particle properties. Instead, it reorganizes existing principles from gravity, quantum information, and thermodynamics under a single geometric capacity constraint that any underlying theory must respect.