Please have a look here:

https://www.eurail.com/en/eurail-passes/eurail-mobile-pass/getting-started

In particular the section "Getting ready to board the train”.

There are mandatory seat reservations to add on some trains in some countries, mainly Italy, France, Spain and cross-border routes.

In Italy :

- 13€ Frecciarossa

- 3€ IC trains

Those are best bought on tickets.oebb.at (no booking fee) : add Interrail as a discount and select one-way tickets.

There are always slower reservation-free alternatives.