The key differentiator between the three Shimano cleat options is the amount of float each allows. Float refers to the back-and-forth movement of the foot while you are clipped in and is essentially how much you can point your toes left or right while your shoe is attached to your pedal. Different levels of float help accommodate different riding styles or help with biomechanical issues such as lack-of-flexibility, past injuries, or knee pain. While float can help reduce stress on joints and make for a more comfortable pedaling movement, it can also result in a feeling of slightly less power transfer if your foot moves around too much.
Your neutral foot position is also important to understand before selecting and installing the right SPD-SL Cleat for you. The neutral position of the foot is defined as its most natural or efficient angle while pedaling. For some athletes, this means that both feet point perfectly straight forward. For others, the toes naturally point slightly inward or outward. If you’ve had an injury on one side of the body, your neutral position may differ between your left and right feet.