I'm afraid you are mixing up some terminology.
You cannot book trains via the Rail Planner App. You do register your journeys there.
See it as your travel diary, which connects to a digital pass. Just before travelling, you register you register your journey, which then can be added to your pass. It asks if you want to activate a travel day. Once added and activated, you have a valid ticket in the form of a QR-code. Each journey (each train you take) should be registered in order to have a valid ticket.
You can add or delete as many trains to your pass if you want. Note that a travel day, cannot be deleted on the day itself. Only activate a travel day just before travelling is the general advice we give here.
There are three broad categories of trains:
1. Trains with mandatory seat reservations (usually high speed trains or long distance trains between big cities in (or to/from) France, Spain, Italy, and operators like Thalys and Eurostar)
2. Trains with optional reservations. Cheaper reservations, always garanteeing you a seat on a train. But you can take these trains without seat reservation too. Mostly trains in central Europe (Germany, Switzerland, Austria,... ICE, EC, Railjet,... )
3. Trains without any form of reservation (the majority of all trains, usually regional trains). Register your journey, add it to the pass. Board, find a seat, and go travel. (REG, RB, RE, S-trains, R, L, Sprinters,. TER, Cercancias,. .. many different names for regional trains)
Reservations are completely separate from the app. Use the reservation module on the Eurail website or use the community made guide:
https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105
If you should miss a connection, because of a delay, to a train with mandatory seat reservation. Speak with a member of staff, they should give you a seat on the next possible train connection to your destination.
If you miss a connection to a train with optional or non-reservable trains. Just hop on the next one, and don't forget it to register it to your pass.
What does your travel plan look like? The community is always happy to give you good advice on where to get your (possible) seat reservations.
