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 also required on your journey, you will need to book and pay for a seat reservation separately. The ticket inspector will ask to see your seat reservation when you show your ticket in the app.
If seat reservations are not required, you can choose any unreserved seat on board (except in 1st class if you have a 2nd class Pass). Adding a journey to your Pass does not mean you have an allocated seat.