Hello PfeiferChi
On Swiss trains, there is usually free seating, this is why your ticket does not have any seat assignments. Swiss trains are a form of mass transit, similar to suburban trains in other countries. As a result, you are welcome to take any vacant seat in your respective travel class. If you want to sit next to someone you don’t know, it’s courteous to ask if the seat is available before sitting down.
You will see once you are here how easy it is to use the Swiss public transport.
Should you reserve a seat? Except for certain panoramic trains (PE), seat reservations on Swiss trains are completely optional and not very common. Swiss people usually make seat reservations when they would like to eat in the dining car, when they travel as a group of 10 or more, or when many travelers are expected. If you still want to reserve your seat, kindly note that this is possible for IC, ICE, EC, IR, RJX, and TGV connections only: Seat reservations in trains and dining cars | SBB
→ Should a seat be reserved, it is indicated above the seat by either a yellow strip or a digital display.
→ Should you travel as a party of ten or more, a group reservation is recommended: Group tickets | SBB
What concerns the Luzern-Interlaken Panorama Express, I, personally, suggest that you should be there 10 minutes before departure latest and to take seats on the right-hand side when facing the direction of travel. The right-hand side offers better views.
Kind regards,
Roland