The Difference Between Overreaching and Overtraining Syndrome

Overreaching can be remedied in a matter of days or weeks with appropriate deloading. Recovery from overtraining syndrome requires at least several weeks, more often several months. So how do we know the difference? How do we know to hold back for a little bit longer if it's a more significant overreaching? And howDo we know to pull back for even months if someone's really in the top spot?