reservation on railjet trains is only optional. If you want to have 100% sure a seat you can reserve a seat, if not you will have to enter the train and find a seat that has no reservation (small screens gives you the information). 

If you want to reserve than do it directly via ÖBB or CD with the seat only option (cost less then via Interrail + seatmap)

Exactly that-much the same as on RENFE cercanias or regional-except that on this train you can also REServe. RJ are very nice trains, very new+comfy and have special cars for also families-with playground and for silence=keep your mouth shut and fone off. Espanoles may not be used to the latter…...