You are allowed to take any route the travel planner gives you to travel between Purmerend and Schagen, so also via Zaandam. As long as you only check in in Purmerend and check out in Schagen, or vice versa, the entire journey will be covered by your subscription.
But if you want to travel to intermediate stations, then only the stations on the shortest route (or up to 10 tariff units longer, which doesn’t apply here) are covered. So if you check in or out in Zaandam, for example, the journey Purmerend-Zaandam isn't covered and you will have to pay for this. If you check in during off-peak hours, you pay the discount fare, else you pay the full fare.
This also applies to stations in Amsterdam. Or any other station in the Netherlands, for that matter. You will always be charged for the shortest route from or to that station and the one on your trajectory. E.g. if you travelled from (checked in at) Purmerend to Alkmaar (where you checked out), you'd be charged for Heerhugowaard-Alkmaar, since this is the shortest route from Alkmaar to a station on your trajectory.