There is no need to worry:
Once you add a pass to your app you create a TRIP linked to that pass. This is your diary of JOURNEYS, and each pass only has one Trip, even if you break it into more than one excursions from home.
To explain by example - I am about to have a holiday using 5 days from a 10 day in 2 months pass. I will then return home for a few weeks before flying out for another holiday using the last 5 days of my pass. All ten days are recorded in the single Trip attached to that pass.
A journey is any excursion you plan to take, normally, but not necessarily so, linked to a single travel day.
There can be any number of individual or connecting trains per journey, and you can have unlimited journeys in a day.
Just like a diary your Trip is extremely versatile and journeys can be added or deleted throughout the period of your pass.
You then add your journeys to your pass by transferring them from your TRIP before you board the train. If as you are travelling you want to board any train not currently in your trip, you simply add them to your trip, either from the planner or by manually adding it if it doesn’t show in the planner.
That will do for now, but I am sure others in the community will send info on reservations and in/out days.