More details are needed to give any sort of precise answer, there are many different railways in Europe.

You can read this comprehensive guide to passholder reservations for most railways.

https://www.seat61.com/interrail-and-eurail-reservations.htm

thank you for your reply. The guide was helpful. Through Eurail website i couldn’t reserve seats on the 6 January 1159-1751hrs Amsterdam to Berlin route on global pass. I understand its not mandatory to book seats but it gave me the option of purchasing seats, but when i got to the final part of reservation an error came up saying i couldn’t book that leg. However so far  going through to the DB website it seems to be letting me. In the DB website with the seat reservation, there’s no place to select that i have a global pass. Does this not matter? thanks 

Tickets don’t include seat reservations so everyone wishing to secure a seat goes through the same process : select seat reservation only and that’s it. It should cost 4.90€ per person in 2nd class (5.90€ 1st), including a seat map.

Possible to pay less through OBB (Austrian Railways) : 3€ (free in 1st class) but no seat map → the system will still assign you the best seats possible. In any case a lot less than what Eurail is offering (when it works!).

I’d make reservations as Saturdays are always busy. :)

As a tip : the front/rear ends are always less crowded and more enjoyable !

okay thank you. Silly question probably but If my eurail global pass is 2nd class, I assume i must book seat reservation in 2nd class only? 

Yes, a 1st class reservation is no good without a 1st class ticket. the pass is your ticket..

thank you,  you’ve been so extremely helpful!

Yes 2nd class only. :)

I think you can get an upgrade (likely expensive) but it’s not worth it for this journey where 1st class is in compartments (old school) while 2nd class uses open seating.