The "R" value is an indication of the level of modulus of the carbon fibers used in frame construction. The higher the value, the higher the modulus.
Modulus simply refers to the stiffness of the individual fibers in the carbon matrix, which will dictate the overall stiffness and efficiency of the material. As the modulus increases, we are able to change the production process to produce a frame of equal or greater stiffness than that of a frame utilizing lower-modulus fibers.
As a general rule, each higher level of carbon will decrease frame weight on average 150-200 grams, and provide a stiffer, more nimble ride.