DAYtime trains have only optional RES. IF your train is so full its standing-then change into other one at some point where more trains are available. Use planner-better those of sbb or oebb-and cut up in segments (for this an old fashioned printed timetable would have been much more informative).

ONly the 1 and only NIGhttime train must be REServed-and you cannot board-will be evicted, if not. But it makes no sense to use NightJet for this trip-passes I halfway night-it is meant for onward major places.

You need to check what the information on ÖBB says about the train.

Both DB and ÖBB add information on certain trains that the demand on the train is so high that you need a reservation to travel, even though reservations normally are optional. 

What date and time do you want to travel?