Structural Selection in Cavity and Resonator Systems: Boundary-Defined Admissible Modes

Resonant behavior in cavities and bounded media is a central concept across classical and quantum physics, commonly described in terms of driven oscillation, constructive interference, and energy reinforcement. In this work, we present a complementary, structure-first interpretation in which resonant modes arise from boundary-defined constraints that select admissible standing-wave configurations prior to sustained dynamical excitation. We show that cavity modes are determined by geometric and material boundary conditions, independent of the specific driving mechanism, while dissipation acts to suppress non-recoverable configurations rather than generate resonance. Discreteness and robustness of resonant spectra are thus understood as consequences of structural admissibility rather than energetic amplification. This perspective preserves standard resonator theory while clarifying the conceptual distinction between mode existence and mode excitation, providing a unified interpretive framework applicable across classical and quantum resonant systems.