selectivity factor, \(S_{f}\)

https://doi.org/10.1351/goldbook.S05567
A quantitative representation of selectivity in aromatic substitution reactions (usually electrophilic, for monosubstituted benzene derivatives). If the partial rate factor, \(f\), expresses the reactivity of a specified position in the aromatic compound PhX relative to that of a single position in benzene, then the selectivity factor \(S_{f}\) (expressing discrimination between p- and m-positions in PhX) is defined as: \[S_{f}=\log _{10}(\frac{f_{p}^{\text{X}}}{f_{m}^{\text{X}}})\]
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1162 (https://doi.org/10.1351/pac199466051077)