On top of the pass you indeed have to add some fees :
- seats on fast/high-speed trains in Italy, France, Spain, Sweden + some cross-border routes
- seats, couchettes or sleepers on night trains
Depending on where and when you'd like to travel, some trains may need to be booked ASAP as they're very popular and/or have a quota for passholders (Eurostar, Thalys).
There are also optional seat reservations on other trains, they do not cost much and are generally recommended in summer (you're always allowed to board but you may have to stand).
- 3€ per train through ÖBB (multiple countries)
- 4.90€ per journey from/to/within Germany through DB
You can often check occupation on the companies websites
Have a look at these links and do not hesitate for questions :
https://www.seat61.com/interrail-and-eurail-reservations.htm
https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105