This publication presents the physical layer of the Theory of Manifold Interfaces as a regularized effective interface-field research program. The central claim is that geometry determines the domain of admissible physical transitions, quantum theory determines a distribution over that domain, and an interface field mediates the transition from distributed possibilities to structured physical regimes. The work introduces a minimal interface field, an effective action, a regularized space of geometries, an interface distance between geometry classes, a physically normalized interface decoherence coefficient, recovery conditions for general relativity and the Standard Model, an observable matrix, and a scheme for experimental and observational constraints. It is not presented as a completed quantum gravity theory or final Theory of Everything, but as a structured effective model with explicit definitions, limitations, testable hypotheses, and open problems.