The Nightjet from Vienna to Amsterdam always need a reservation.

If you travel with daytrains no reservation is needed but sometimes it´s better to reserve.

Dont trust the Reservationpage of Interaril/Eurail

For Nighttrain you can have a look via oebb.at if you can reserve a seat Add Discount Interrail/Eurail and then click on Single Daytickets and not on Seat only

For daytrains route i recommend to reserve via bahn.de (Seat only) as they charge just 4,50 in 2nd class per journey (doesnt matter how many trains will be catched)

You can always-but without garantee on even to be seated, use the DB Night ICE from München @ 0.03 to Köln-Düsseldorf for onward to NL and check what train you have to take from Graz to reach it PLUS allow at least 1 hr extra as Bayern Grenzpolizei checks these trains in a time consuming process @ Freilassing. I´ve done that several times in various variations. IF you have the  whole day-on FRI transit is free in Salzburgland-so you can use the trolleybuses for some sightsee. But as 0.03 is next day, you need 2 traveldays if you have a pass like that.

For day trains, use the DB website to book reservations (€4.50 per journey). For the Nightjet, use ÖBB. Both much more reliable than Interrail.

Thanks for helping me out. I made a reservation for a day train on DB, so I think I'm ready now.

Someone told me the connection with Interrail should be mentionend on the reservation. I guess this can't be true, because it was not something I could mention when making the reservation.

You have to mension Interrail only for Reservations for trains with compulsory reservation, so you can get this reservation, because normaly this trains have the reservation included in the Ticket.

On trains with optional reservation you take a normal reservation if you want a reservation