relative retardation, \(R_{\text{rel}}\)

in planar chromatography
https://doi.org/10.1351/goldbook.R05276
A term which is equivalent to relative retention used in column chromatography: the ratio of the R F value of a component to the R F value of a standard (reference) substance. Since the mobile phase front is common for the two components, the R F value can be expressed directly as the ratio of the distances travelled by the spot of the compound of interest (bi) and the reference substance (bst) respectively: \[R_{\text{rel}}=\frac{R_{\text{F}\left(i\right)}}{R_{\text{F}\left(\text{st}\right)}}=\frac{b_{i}}{b_{\text{st}}}\] In former nomenclatures the symbol R s was used to express relative retardation in planar chromatography. Because of its identity with the symbol for peak resolution the symbol R rel is suggested for relative retardation in planar chromatography.
Source:
PAC, 1993, 65, 819. 'Nomenclature for chromatography (IUPAC Recommendations 1993)' on page 845 (https://doi.org/10.1351/pac199365040819)