I'm guessing English isn't your first language, but that's okay.
If you're asking about the damage you do to the legs, I am not sure if the damage values are different or not, because I have very rarely seen a situation where the outcome would change.
If you're asking what happens when someone jumps on top of you and you knife them in their heels, it actually counts as a backstab, which does more damage. However if you're hitting the front part of his foot maybe it does not count as a backstab. This can happen in Nuke's vents very frequently, and I don't know if it does less damage sometimes if you shoot someone on their toes, I have not tested it or have any experience on it.