A quantitative representation of @S05563@ in @A00441@ @S06078@ (usually @E02020@, for monosubstituted benzene derivatives). If the @P04421@, \(f\), expresses the reactivity of a specified position in the aromatic compound PhX relative to that of a single position in benzene, then the @S05563@ factor \(S_{f}\) (expressing discrimination between p- and m-positions in PhX) is defined as: \[S_{f}=\log _{10}(\frac{f_{p}^{\text{X}}}{f_{m}^{\text{X}}})\]