According to Nomenclature for Chromatography (IUPAC Recommendations 1993) [1, pp. 843, 845] (also listed in IUPAC Gold Book), retardation factor is denoted as follows:
Column chromatography: $R$ (capital $R$ in italics).
3.7.13 Retardation Factor $(R)$
The fraction of the sample component in the mobile phase at equilibrium; it is related to the retention factor and other fundamental chromatography terms:
$$R = 1/(k + 1)$$
Planar chromatography: $R_\mathrm{F}$ (capital $R$ in italics and upright capital $\mathrm{F}$ in subscript).
3.8.04 Retardation Factor ($R_\mathrm{F}$)
Ratio of the distance travelled by the center of the spot to the distance simultaneously travelled by the mobile phase. Using the symbols of Fig. 2:
$$R_\mathrm{F} = b/a$$
By definition the $R_\mathrm{F}$ values are always less than unity. They are usually given to two decimal places.
In order to simplify this presentation the $hR_\mathrm{F}$ Values may be used: they correspond to the $R_\mathrm{F}$ values multiplied by 100.
Ideally, the $R_\mathrm{F}$ values are identical to the $R$ values (see 3.7.13).

Reference
- Ettre, L. S. Nomenclature for Chromatography (IUPAC Recommendations 1993). Pure Appl. Chem. 1993, 65 (4). DOI: 10.1351/pac199365040819.