ehh, yes and no. No because it kinda doesn't but yes because you wouldn't put a 90 "flex" boot on a 230 pound man, he'll flatten that shit like a pancake idc how soft he likes his boot I wouldn't sell him a "soft boot" because there's no point, he might as well be clicking in with his sneakers on and just lace em tight lol.
And the word "flex index" is made up, there technically is no INDUSTRY STANDARD flex index and the reason for that is because each company measures flex differently. For example the a lange rx 130 flex's and feels extremely similar to a demon 110. But there different flex index's. Ski boots are all about how the boot fits you. You shouldn't have to crank down a boot for it to hold you where you want to be. The better the boot fits you the less important flex is because the boots doing its job.
But yes, you can use flex to get a GENERAL IDEA for what your looking for.