exempt vs non-exempt
In the private sector letting non essential employees go is called a lay off. In the rare event they are re hired they dont get back pay for the time they werent working because they werent working. Why would an employee who is not essential have a job in the first place?
No, that isn't the distinction. You had it right the first time - Essential vs non-essential. A non-exempt in a critical area may have to show up, while a non-essential exec may sit at home.