This work presents a structural interpretation of spectral observations in terms of degrees of freedom (DOF) and constraint-limited accessibility. Rather than treating spectra as continuously preserved and attenuated records of emission, we interpret observable spectral structure as the subset of modes that remain admissible under constraint. Empirical analyses of spectral datasets reveal that line accessibility evolves through discrete regimes characterized by threshold activation, mutual exclusion, and localized reductions in accessible modes. These features are not consistent with uniform scaling or gradual signal degradation. Instead, they indicate that observability is governed by constraint-limited accessibility. Within this framework, spectral degrees of freedom are defined as the number of independently accessible modes within the observable wavelength window. Spectral evolution is therefore understood as transitions between admissible mode configurations rather than continuous transformation of a fixed set. This interpretation extends standard descriptions of redshift and attenuation by introducing accessibility as an independent observable constraint. Classical scaling and attenuation remain valid within regimes, but do not determine which modes are accessible. This paper serves as a conceptual companion to empirical studies of spectral accessibility, providing a structural framework for interpreting discrete spectral regimes, mode switching behavior, and constraint-induced suppression of observable features.