Is adding a journey to my Pass the same as a reservation?
No, adding a journey to your Pass is not a reservation. If reservations are required on your journey, you will need to book and pay for a seat reservation separately. For trains that require reservations, the ticket inspector will ask to see your seat reservation along with your ticket in the app.
If seat reservations aren’t required, you can choose any free seat on board (except in 1st class if you have a 2nd class Pass). Adding a journey to your Pass doesn’t mean you have an allocated seat.