關於愛，我是個小學生。

International Flex Code is the invention of Miyazaki shafts with the 4 digit number relating to each part of the shaft: butt, mid-butt, mid-tip and tip. Each digit of the code corresponds to the relative stiffness on each part of the shaft - 9 being xx stiff and 1 being something around ladies or seniors flex.
And therein lies the problem.
It's one manufacturers interpretation on what is flexible or stiff based on a rating of 0-9. It could be no different to using colours of the rainbow, or dare I say it, the terms "regular" and "stiff". Without a quantifiable measuring process used to ascertain flex, frequency and the like, it remains a PR tool to Miyazaki. It doesn't give the end user any idea whatsoever how shafts will play compared to next in real life - let alone on how they will feel.
Hence the IFC is about as much use as the "older" R,S and X etc. along with the equally baffling torque rating.