This paper develops a theory-neutral structural result concerning relational descriptions in physics. Many modern frameworks assign physical meaning through interactions rather than observer-independent intrinsic properties. This raises a general composition problem: if facts are generated locally within interaction contexts, what determines their coherent comparison across later chains or networks of interactions? Within a minimal framework of interacting systems, locally assigned relational facts, and maps between fact contexts, the paper establishes an interface necessity theorem: local fact assignments together with bare accessibility structure are not, in general, sufficient to determine unique cross-context composition outcomes whenever later comparison, inheritance, or aggregation of records is permitted. Some additional structure governing admissibility, translation, filtering, compatibility, or equivalence between relational fact spaces is generically required. This additional layer is termed interface structure and is used operationally rather than ontologically. No new force, hidden variable, or replacement dynamics is proposed. Instead, the result identifies a structural requirement linking local relationality to broader coherence. The paper discusses implications for relational quantum mechanics, decoherence-based narratives of record formation, Wigner-type multi-observer scenarios, and broader network descriptions of physical information. This work is related to the broader Horizons-as-Dynamical-Interface Framework (HDIF) program.
Chaim Zeitz (Tue,) studied this question.