I have a 2nd class Eurail pass. I am looking for a train from Interlaken to Paris. And in the Eurail site, part of the journey is Reservation optional and another part is Reservation mandatory. I know it seems obvious, but I want to confirm. In the reservation optional part of the journey, do I just go and sit on any seat in the train?

EDIT

The reservation optional train is IC 1058 INTERCITY from Interlaken to Basel SBB