What constitutes an update? - Feeds

One of the pricing metrics for the monthly Stream Feeds plans is updates. But what is an update?

When a user sends an activity to any type of feed - that is an update. But if the feed is followed by 100 other feeds, then they will also receive the activity on the following feed due to the fan-out process. This means the single activity will constitute 101 updates. 

Other things that count as an update:

1. Following a feed (0 if activity_copy_limit is specified as 0)
2. Unfollowing a feed (unfollow will count as 25 updates)
3. Removing an activity from a feed
4. Retrieving a Notification Feed