A Schengen C visa can be issued for specific member states or for all member states. The latter is done by writing "Schengen States" in the national language of the issuing state.
With a single-entry visa for "Schengenstaten" you can adjust your original itinerary, even to visit another Schengen state. For example, if you initially planned to fly to Schiphol and then spend two weeks in Amsterdam, you can instead decide to fly to Paris and take a train to Amsterdam instead. You might even spend a day in Belgium en route. The Netherlands would still be the main destination of your trip. You could not decide to go to Iceland instead.
With a long-validity, multiple-entry visa, the first visit should be roughly as described in the application. Again minor adjustments are possible, but a completely different itinerary would appear like fraud. On subsequent visits you are free to visit other Schengen states.