Reactivity is at the upper limit of frequencies available. It’s just a name, but we tried to differentiate it a little from the other explosive force settings. The belief is that if, for instance you need to keep jumping as for basketball rebounds, you need to be able to react very fast.
I may not be 100% correct, but since in a sprint you need to develop high force for a certain amount of time, in that case you need to develop the highest power. Maximum power is developed by muscles not at their maximum speed but somewhere between 1/2 maximum speed and 1/3 of maximum speed.
I’m not a specialist, so you need to verify this information with a competent coach like Charlie and others in this forum, because there may be other factors.