Yes you need to book accommodation : seat, couchette or sleeper. Go to nightjet.com and add Eurail as a discount.

Night trains are very popular so do not wait to book. For questions mention your travel date.

Thank you ! 
Also discount offer is for interrrail/ eurail - global pass . I guess since it’s mentioning Eurail Globalpass it is for me ? 

Thank you 
 

Yes that's the one. With a Interrail/Eurail pass you only pay the accommodation so a seat, couchette or sleeper.

Great , because my pass is my ticket to trains . I definitely need to book that . Excellent! 
Should I also book my regular trains like Salzburg to Zurich, or Zermatt to Zurich?  It’s my first time using Pass . I always used HalfFare card for Switzerland. But since I will have it should I book seats for other trains ?
thank you 

Seat reservations are really not a thing in Switzerland, often not possible anyway. You simply add the train to the app like usual (in order to get a QR code = your ticket) and hop on hop off as you wish.

Salzburg - Zurich has optional reservations, recommended in summer and on busy days : 3€ on tickets.oebb.at

Thank you !!!

Hello , can I use Eurail   global Pass for Westbahn train from Vienna to Salzburg? If yes , do I need reservations for seat ? Thank you 

Natalie 

Yes, you can use your pass on Westbahn but only in 2nd class even if you have a 1st class pass.

You don’t need reservations but can get them at https://www.westbahn.at/

OBB also serve that route, again reservations not necessary but possible at www.oebb.at a better option for 1st class passholders.

Thank you for answer.. ! OВВ  I think has faster train on this route if I’m not mistaken. And I have 1 st class which is great .