In THIS case and for this trip: nothing. The only thing you have you have to do-but that should be obvious as its always needed, is to activate the passday and then add this trip. You can add-if so wished, more trips later the day but only IN Österreich-Salzburg is considered a border point,as from there also German fares apply (ONly on that line toward DE), so your Austria-only pass would not be valid for onward then (that is meant by conditions)

(BTW: advance fixed fares for this line start from 9€. THere are also WESTbahn trains-from the other old West station in Wien. No worries if you miss this train-they run 2/hr.

Thanks so much for the very helpful info! So glad to know that we are good to travel on this train to Salzburg with our Austria travel passes. So that means as long as we do not travel beyond the Austrian borders, we are fine. 

I have bought first class passes. We will be travelling end November to mid December to Vienna, Salzburg, Hallstatt and Innsbruck. Is there a need to reserve seats as i understand that there is only 1-2 carriages for first class and seats may be limited? 

If you intend to travel between 8 and 11 December, then for sure reserve a seat (if you travel on the Westbahn). Otherwise it is not neccessary (except Friday and Sunday afternoon). 
There are between 1,5 and 2,5 1st class carriages

@mcadv please show me the 9 euro fares from Wien to Salzburg! Because there are NONE. Don’t even know if there are still 9 euro tickets to Linz, but to Salzburg for sure not.

Also the other info from mcadv is wrong. It is not because the train goes to Salzburg but it is because the railplanner app is full of wrong data. This train is written as EC (although it is an RJ) and due to being an EC (EuroCity) the app thinks it is an international train (even if it is not) and says that conditions might apply...