F series: This is the smallest chair that Dxracer offers, the "standard chair". It is optimal for "normal weight" people up to 175 cm and 100 kg.
The " F " here stands for Formula Series.

R-series: The R-series chairs are particularly suitable for taller, "normal weight" people: 170 cm to 190 cm and 100 kg.
The " R " in this case stands for Racing Series.