The pass covers any train except a sleeper service.
You are paying for flexibility with that pass, the ability to travel on any train on your eight travel dates. If you have fixed your itinerary (and you probably will because you'll have hotel reservations), you can travel for less than the pass by using "advance" rail tickets. The name "advance" is a particular ticket type, cheap fare but no flexibility, and yes - you do have to buy it in advance.