Huldildeu54
You probably the Saver Day Pass.
You don’t ‘book’ a route when buying a ticket, you pay an admission fee for the transportation service and the ticket is the proof for that.
Note that the Bernina Express (marked as PE in the timetable) has a few regular coaches attached without reservation. The latter give you the flexibility to e.g. interrupt the journey at Alp Grüm or at the Bernina Pass and take the following train, something those pitiable folks with their reserved seats in the panoramic cars can’t do.