A relativistic quantum concept defining wave equations and state representations consistent with relativistic kinematics. It governs spinor structure, dispersion relations, and coupling to electromagnetic potentials when included. It does not replace full field-theoretic treatment for particle creation processes and is applied within stated approximations. It is used to model high-energy or high-velocity regimes and to derive effective low-energy limits for certain systems. The concept is generally stable, though formal derivations and numerical methods evolve over time.