The stiffness index rates how much the sole resists bending under your pedaling force. A higher number means a stiffer sole that transfers more power to the pedals, while a lower number means more flex and comfort.
Here's the catch: each brand uses its own scale. Shimano goes up to 12, Sidi up to 14, and the numbers don't translate directly between manufacturers. As a rough guide within any brand's range, 1-4 is recreational, 5-8 is sport-level, 9-12 is race-level, and 13+ is ultra-stiff pro territory.
Most recreational and fitness riders are well-served in the 5-8 range. Going maximum stiffness isn't always better—ultra-stiff soles can cause hot spots and foot fatigue on longer rides, especially if you're not putting out race-level watts. Choose based on your riding intensity, not the highest number on the shelf.