A quantitative representation of @[email protected] in @[email protected] @[email protected] (usually @[email protected], for monosubstituted benzene derivatives). If the @[email protected], \(f\), expresses the reactivity of a specified position in the aromatic compound PhX relative to that of a single position in benzene, then the @[email protected] factor \(S_{f}\) (expressing discrimination between p- and m-positions in PhX) is defined as: \[S_{f}=\log _{10}(\frac{f_{p}^{\text{X}}}{f_{m}^{\text{X}}})\]