Hello EvaJa
The greatest advantage of the Saver Day Pass is, it grants unlimited travel on the Swiss public transport system for the day of validity, also in cities. Mountain excursions are excluded, with some exceptions.
You can find its validity map on the Saver Day Pass product website, in the Downloads box:
https://www.sbb.ch/en/tickets-offers/tickets/day-passes/saver-day-pass.html?lang=en
As our trains are free seating, you can simply board a public transit service at will, and hop on/off where ever you want/need. You do not need to notify the train operator about your planned trip ahead of departure.
When you get on a train, you look for the next free seat in your travel class.
More information on seat reservations here:
https://community.sbb.ch/blog/23887-demystifying-seat-reservations-on-swiss-trains-a-comprehensive-guide
Kind regards,
Roland