The Ticket is your QR-code. Don’t generate this until you’re very sure you’re taking the train. I always wait until I effectively see the train before generating the QR-code in my pass. You never know if your plans change. Now you can still delete that travel day. But once the travel day has started, you cannot delete it!
Some trains require mandatory reservations, especially night trains, Eurostar and long distance high speed traffic (TGV, AVE and Frecce trains). You just cannot show up and board the train with a valid qr-code then. You will need a reservation, which mentions the exact train, which carriage and seat you have been allocated (or bed in case of a night train). It comes in a pdf and needs to be shown to the inspector. On night trains this needs to be printed, because your carriage attendent will keep it. If it has been sold out, it’s sold out. You won’t be able to travel as a standing passenger. You will need to find another connection then.
Indeed: all reservations are explained in the link below. Please read this carefully.
Night trains are very popular, and capacity very limited. Chances exist that proper beds might’ve been sold out. In that case, just take day trains to Vienna. It can be done with one switch of trains in Frankfurt from Brussels. I generally find day trains way more comfortable than night trains. Although it’s nice to save up on a hotel day.
And make sure to skip Thalys. A seat reservation for pass holders costs more than their cheapest tickets!
From Belgium I see @manond2896 , Francophone of Nederlandstalig?