Complex sentence: Added under a horse's natural hoof, pads (sometimes called "stacks" or "performance packages") can vary in height; they may be up to thick in the heel and no more than thick in the toe.
Split into simpler sentences: Added under a horse's natural hoof, pads (sometimes called "stacks" or "performance packages") can vary in height. Pads may be up to thick in the heel and no more than thick in the toe.