You might not have a seat.
In European trains, the seat reservation is independent from the train pass. Some trains require a seat reservation (TGVs in France) but not all. In German countries (at least Germany and Austria) it is possible to buy a train ticket and then a seat ticket. You can also buy it anytime (as long as there are seats left) and the price is always the same (apparently 3-3,5€ in Austria)
In practice, what will happen if you don't pay for the seat is that you're gonna go through the coaches and search for a free seat. In Austria, it apparently is written on the seat whether it is reserved. Most of the time you'll find a seat, since trains have hundreds of seats, but you might look for some time.
For long-distance trains (like ICEs across Germany) the seat will be used by several passengers throughout the trip so you might need to switch seat if it is reserved for a short part of the trip.
Another important parameter is peak hours. I suppose the train will be pretty full then. Especially on Railjet trains, that are apparently operating on the main lines of Austrian network.
For more details: