When traveling by train in Europe, there are two components: the ticket and the seat reservation. A rail pass serves as your ticket.  

If the train you're taking requires a seat reservation, then you must have both a valid pass (ticket) and a reservation to board.  

Even for trains that don’t require a reservation, you can still choose to reserve a seat—especially for longer journeys or if you want to guarantee a seat for a more comfortable and stress-free experience.  

However, some regional trains (local trains) do not offer reservations at all, so they are always open seating.