Great question. I also wish this was documented somewhere. Maybe it is - but I have never seen it.
Not sure where you saw "active endpoint" but an endpoint is not counted as being active. It either exists in the database (MAC address and all its associated attributes) or it doesn't.
Active Sessions , on the other hand, is an authenticated endpoint that is being managed/tracked by ISE as a session from the time the endpoint was authenticated, and the session's life is tracked by means of ongoing radius Accounting, or if no radius accounting is seen for that endpoint, then ISE will eventually terminate the session (I think it's a few days).
