Reservations and app are separate. In the app, you still have to add the journey yourself, like you would do on a paper pass in the travel diary.

Currently, the app is just a replacement of the paper pass and reservations stay separate.

Ok thank you, so does this mean that the travel day was used if I reserved it via the website? Is there anywhere to see that? 

Also, when on the website I do not see the reservation anywhere ? But I was emailed the confirmation with pdf tickets

edit: I do now see the reservation on the site but still wondering how to see the used travel day somewhere

AFAIK you don't see it on the website. It's the app that keeps track of your travel days.

okay but I booked the reservation or this train and it is not on the app. So theoretically I can just not add this travel day and it won’t count? That is what I am not understanding. Is it the QR code that comes that I 100”% need which would make me need to enter it? Then the reservation is at another counter when I am there? 

Conductors check TICKETs-and sometimes, if it also includes a kind of supplmt (in that case it would apply to any passenger, not just passholders-that is the case on some trains on some railsystems) the extras like a REServ.. In well over 80-85% of trains anywhere in EUR you only need a ticket-much like in the UK too! In most long-dist ¨InterCity´ trains one can reserve, if desired. And those hi-speed trains in FR/IT/ES (and thus also this €*) MUST be reserved-so around half of the items in this forum concern that-and noone even bothers to realise that everything has already been asked before..

Now in that €* its a bit different as they like to imitate airlines so much, but as its a commercial enterprise and they do have to watch their income, they will certainly insist in seeing to it that you have paid=filled out a passline/used the app correctly. BTW-it may even well be if Lon-Par is the only trip to do on a day, that cost of passday+extra suppmt (30€) is higher as a normal ticket.

Right now-after some 3 yrs again I have arrived in this UK and plan to do a tour with an InterRail tomorrow-I just use the pass and see what seat is ´available´-and hope&pray that not too many trains are cancelled for whatever reason as I noted today

You need both: you need to add the journey to Myn Trip and then My Pass. This makes you have a travel day to use and you have a valid ticket. For Eurostar (and some other trains, mostly highspeed or night trains) you need a reservation in addition to your pass.