For general X-ray absorption spectroscopy (XAS) or X-ray absorption fine-structure (XAFS) analysis, whether focusing on the pre-edge, the X-ray absorption near-edge structure (XANES), the standard full XAFS region or the higher-energy extended XAFS region (EXAFS), a series of important parameters come into the theory across the whole International Tables for Crystallography. For near-edge and pre-edge structure, the definition of the Fermi level E_F is critical. The ionization hole width Γ_H and inner-shell hole excited-state lifetime τ_H are another pair of critical parameters that limit the information content of the near-edge spectrum, revealing key physics. The Debye temperature Θ_D represents the complex dynamical behaviour of electrons in the quantum system. Much less understood is the damping of the signal due to the photoelectron wave inelastic mean free path λ_IMFP, the electron energy-loss function, the complex and anisotropic refractive index n and the complex and anisotropic permittivity or dielectric function ɛ. Here, an attempt is made to give a brief summary of the key issues and typical values, and also a discussion of scales and units for representation of key data and critical parameters.