Outsole stiffness is optimized for different riding uses.
More rigid options prioritize power output and less rigid options allow for greater walkability.
For example, the stiffness of the RC9, which is made for professional road racers, is rated at the maximum 12 on our own Stiffness Index.
On the other end of this index with a stiffness rating of 2, are shoes like the GR5 and CT5 that are intended for a wider range of riders that spend more time off the bike.