You should activate the pass from your first travel day, 7th of June.

If you for some reason don't start your trip on the 7th of June you can change the activation date until 23.59 CET on the 6th of June.

 I understand , so I can not book my trains yet and pay for seat reservations ?

Yes you can buy reservations on interrail.eu website before activating the pass. You can’t buy the reservations in the app anyway — the two processes are separate. I bought my Eurostar reservation in March, but I only just received my new passport, so I activated the pass just now, for July (the date of my Eurostar reservation).

It’s really confusing...

I take my first train on June 7, if I activate it now, the validation time starts, right?  and you say that I can reserve my trains and buy through the page but, without activating the pass or can it be activated from the day I take my first train?

You don’t make reservations on the app. You have to go to a website like interrail.eu or some other train website. So, no, you don’t need to activate your pass to buy reservations. You do that before getting on the train (or activate in advance, but if so you activate with effect from the first train date).

(They use the word “activate” wrongly on Interrail I think really. “Activate” is not something you can do in advance, in English)

Example: I have just arrived in FRance today at a fairly big station/gare. I just check the big mahcins and can now buy REServ for TGV etc for in sept (when I have another pass-the 50% discount). So all trains wnated still have the  10€ quota and this is even exchangeable till day of departure.

Thank you for your response, so should I reserve my seats and pay for them through the page and not through the app?  if I do it from another train page I will logically charge the ticket.

The app only forwards to some of the websites where you can book reservations. You can use whatever you like.