Boundary and Interface Systems: Admissibility at the Point of Contact

This paper establishes boundary and interface systems as the domain-level realisation of admissibility within the Paton System. While prior work defines admissibility as the condition for system membership, observation as registration, and continuation as persistence, the location at which these conditions are enforced has not been formally isolated. This paper shows that boundaries function as active constraint interfaces where admissibility is evaluated, rather than passive separations. Interfaces define the structured pathways through which systems interact, while boundaries determine whether such interactions are permitted. Interaction is therefore contingent on admissibility at the boundary. This provides a unified structural interpretation of interaction across physical, biological, computational, and cognitive domains, positioning boundary systems as the operational expression of admissibility.