That said it’s neither of the above. It’s the unique (singular & counted once per period) count of machines that have requested an update in the past week.
That’s why this value goes down as well as up (e.g., over Christmas the whole store drops maybe 10%). A machine will appear as “Active” until the store determines its not checked for updates in a week and then is removed from the tally. This client may have just gone on holiday or not been used for a week, in which case they’ll reappear in the count when they next come online.