You cant book reservation via the Railplanner App it just redirect  you to the website where you can buy reservation :)

If you “just” added the trip to your Pass this is no booking This just generate your QR-Code which is finally your Ticket :)  For Eurostar London - Paris you need both Valid QR Code in the Railplanner App (in case of MobilePass) and a valid Reservation bought via Eurail/Interrail or via B-Europe :)

Just note you can change your Seats or (in case of 1st class) order a special meal on Eurostar.com with your booking reference :)

It's impossible to book seat reservations through the railplanner app. I assume you have made them through the Eurail reservations site?

Aside from the reservation, you will have to add the trip to your pass (and thus activate a travel day). Do this shortly before departure as travel days cannot be cancelled on the day itself. 

Hi your Eurail Pass is your Ticket.

Some trains (mostly local trains) can you just add the trip to your pass and hop on the train. You simply show the QR-Code for Ticket check.

Mostly in Middle Europe most long distance trains have only optional reservations. You don't need a reservation, but you can do it (I reccomend when you want, do it directly at the website of the train company. Eurail.com simply ask to much for the same service.

Some trains (mostly all high speed trains, and in some countries also long distance trains) have compulsory redervations. Whithout a reservations you can't get on.

That’s what i meant! The app redirected to the website. So to confirm as long as I add the trip to my pass (on the app) and activate, I’m good to go? 

Yep the app redirected me to the website is what i meant! Thanks for your help :) 

You are ONLY good to go with app/trip IF that train you take has no or only optional RES. As said several times above-and as you also started with; a RES is needed-and cost money, from 2-3 to >30€, also of course for sleepers in night-trains. Actually it is not that much different from down yundah in OZzie land: Staterail or Vicrail near SYD or MEL would be regional to us: just get on with trip in pass, but between the 2 on long-dist you need to RES.

But the point here is that about each newbee wants to use only those trains in FR and ES that have these pesky RES and are also hard to get. You can even set planner to ´trains without RES´ but then you see it may take much longer. Also many an OZzie gets disappointed here-several the last days, as RES can often only be done till 2 month ahead. So that carefully planned monthes long dreamtrip of a lifetime will get shaken and has to be adjusted.

In case you are still a little confused the simple answer is to travel on any train in the network you first need a valid ticket. With a mobile pass that is achieved by adding the train you plan to travel on to your “Trip”. Then before boarding that train you simply move the slider by the train and this creates a travel day in your pass and adds the train to a list below the QR code. This is now a valid ticket for that train. If you travel on any more trains in the same travel day you add them via your trip to your pass and they will be added to the list below the QR code.

Trains can be added from the App planner by selection or manually.

Now onto compulsory reservations - These are separate from the app (although there is the link you referred to for the Eurail/Interrail reservation tool.) For trains with mandatory reservations you need both your evidence of the reservation as well as the validated pass (i.e. with a QR code and the train added to the list). 

If mandatory reservations (or a rare supplement such as travelling from Italy on the Cross Border ECB trains) are NOT required you do not need anything other than the validated travel day in your pass, although many express and long distance trains have optional reservations. Optional reservations are a matter of choice and are usually relatively cheap.