Hello @petershotts
You can make this check-in only with an e-ticket of Trenitalia. If you buy this e-ticket at a ticket office in another country, you must also give you email address. With this address and the ticket code, you can import the ticket in the Trenitalia app and make the check-in or choose another train in the app.
If you make a reservation for long-distance trains for your Interrail/Eurail pass, you can also import the ticket in the Trenitalia app and manage it, for example you can choose another train. After a change, the reservation is no more refundable.
Thank you. As soon as I wrote my post I realised that I was getting confused and the answer to my (somewhat confusingly written) question is, I assume, that there is nothng that needs to be done!
The point about regional tickets needing to be validated is to stop them being used muttiple times for the same journey. Clearly the interrail pass shows the time of the train you are taking and so the validation issue is irrelevant.
And you can also use your pass multiple times for the same journey.
A ticket inspector confirmed to me that he don’t see the trip by scanning the QR code, he see only if the pass is valid or not (I think that the QR codes are not generated in the app, but downloaded in advance for every day when you activate the pass). But a visual control is always possible.