For which journeys is scanning the QR code in the app meant?

Also when you buy a reservation via App (which only shows the interrail website, not possible via app), you will get the reservation via email. And you will have to show it as PDF on the phone or as print at home. 

Don’t forget to add the trip to your pass in the app before entering the train. 

As a newbie about to undertake my first trip I may be corrected, but I understand that reservations are not directly linked to your pass and will need to be presented separately to your pass or ticket. 

Although many can be displayed on-screen, I intend to carry a hard copy (PDF printout) of all my reservations, although I accept a screen showing the QR codes is often adequate. Any reservations I might make in country would almost certainly be at a ticket office and therefore printed for me in a form acceptable to the train staff..

I have booked reservations through a few different sources including Interrail reservations and all have confirmed with printable reservation details.

@Yorkie you’re right.

Don’t forget to add the trip to the pass in the app or on the paper pass, without your Ticket (=Interrail/Eurail) is not valid. 

So the code in the app is mainly for trips that are added to your journey that did not need reservations? Or will the conductor scan the QR I received in my email as well as the QR in the app (in case of reservations)?

NO- the APP-it will give a code for all day-IS the main thing-that is what conductresses will want to check and is your proof as ´ticket-replacement´. IF you canNOT show that-then you are thought of as being a fraudulent blackrider-and may be subject to all nasty things those people can get. Most often they have no interest to see those RES-things. What you most often need that RES for is to chase the ignorant others from your assigned seat-many will not accept this ´the seat was free´.

Once again - Please feel free to correct me. My simple understanding is:

With a mobile Global pass you have to add every train you plan to board into the app before boarding, and this becomes your “ticket”. If no compulsory reservation is required you can do this at any time before boarding including on the platform. If you have used the app to create a multi-train journey it will add all trains on the journey (I think).

For trains where a mandatory reservation is indicated in the planner you also have  to acquire the reservation before boarding and have a totally separate proof, which is not kept in the app nor linked to your specific pass. For example I already have a Eurostar reservation booking for late August, but do not need to add the actual journey to the app till much nearer the time, as all the advice is to make reservations well in advance for popular trains with limited availability for pass holders, especially Eurostar.

As documented elsewhere there are several ways to make reservations. I have used Interrail and OBB with no problems (yet).