Pitcher fatigue question

[k_grandt]

Is there any formula to understand the pitcher endurance rating? A closer with an endurance rating of 1, is ready to pitch back to back days, while another pitcher, endurance rating of 36, shows worn out after each use. Is the rating based upon how many pitches they throw? Or how often they are used? And, is it one of the ratings that will improve over a career?
thank you