Golf club flex refers to the amount of flexibility in the shaft of a golf club. Different shaft flexes are designed to suit the swing speed and style of different golfers. For example, a golfer with a slower swing speed may benefit from a shaft with more flex, as it will allow the clubhead to accelerate through the ball more effectively. On the other hand, a golfer with a faster swing speed may benefit from a shaft with less flex, as it will provide more control and precision. Golf clubs are typically available in different flex options, such as regular, stiff, and extra stiff, to accommodate the needs of different golfers.
The flex you are familiar with A, R, S, X etc has no real meaning with each manufacturer giving it's own measurement and thus, shafts with the same flex from different manufacturers with the same flex can be wildly different.
At Cool Clubs we use the FM scale to determine shaft flexibility. Like Flex this is a measure of the shafts flexibility but it is much finer grained allowing for more precise and repeatable build of your clubs. It is also not reliant on a particular manufacturer. So if we build a club to say 7.0 FM we know, regardless of shaft brand that we can build shafts exactly to the frequency.