When a user follow your calendar, the users calendar program stores an unique URL which the calendar program request a couple / up to many times each day.

If the users calendar program requests your calendar, the user is actively following your calendar and is marked as a follower.

If the user is inactive for 30 days, we mark that user as an inactive follower. We use a 30 day timespan because the user could be on vacation etc.

The number of total followers is indicating how many user in total have followed your calendar.