The HFC Companion Guide provides a formal reference layer for the Standard Coherence Fidelity Layer (SCFL) and the Upstream Coherence Measurement Stratum (UCMS) as applied to cross-domain coherence measurement. This document clarifies the relationship between: • Foundational physics (state trajectories x(t)) • SCFL measurement operators • Multi-layer structural classification • Comparative retrospective analysis • Operator-specific prospective measurement The framework is substrate-agnostic and designed for application across human, organizational, and institutional systems where timestamped or sequential data exists. Formal notation is specified for: • Structural state space x(t) \in 𝒮 • Drift and acceleration derivatives \dot{x}(t), \ddot{x}(t) • SCFL measurement operator M • Structural code tuple C_s = (T_i, S_j, D_k) • Domain separation 𝒟_{retro} \cap 𝒟_{prospective} = ∅ The guide emphasizes falsifiability, replication, and strict separation between retrospective comparative study layers and prospective operator-specific measurement. No clinical, diagnostic, or treatment claims are made. All artifacts are published under open academic license. Primary measurement reference: Brogdon, R. (2026). Standard Coherence Fidelity Layer (SCFL). Zenodo. https://doi.org/10.5281/zenodo.18622508