The flex of your boot should be based on a combination of your ability, aggressiveness, weight, length of your leg, and ankle flexion (the amount of flexibility of the Achilles tendon). The bigger, stronger or the more aggressive you are the stiffer your ski boots should be, to provide stability, power and rebound.
Very Soft boots are for the lightest skiers and true beginners.
Soft flex boots are ideal for beginners or lightweight intermediate adults that weigh less than 115 lbs.
Medium flex boots are great for bigger or athletic beginners, intermediate skiers of all ages, lightweight advanced adults, or advanced skiers with limited ankle flexion.
Stiff boots are designed for bigger or athletic intermediates, advanced skiers, lightweight experts, or experts who have limited ankle flexion.
Very Stiff boots are for the biggest, strongest, most aggressive skiers that are putting the most pressure on their boots.