All our plans are based on usage. If you are subscribed to the monthly plan, your monthly price depends on your monthly usage. If you are subscribed to the volume plan you have a fixed usage valid for one year.
Usage = Visitors + Requests
But how do we count that usage? That's simple: Every Web app users session and every API requests are counted and your usage is the sum of both.
The user session will be gathered from Google Analytics to guarantee that only real users will be counted and crawler traffic or web scrapers will be filtered.