You can't make reservations in the app.

The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable and availability on the websites of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries. 

The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website. You can look at the guide in the link:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

If you, after having looked at the guide, have questions about how to make specific reservation, please give your travel details (departure date, time and route) preferably in a new post, and you will get advice. 

Your/this pass does NOT include the trip to boirder!

IN FRanc-and mostly for these TGV there are 2 levels: 10€ for IN a small quota and if that is taken (or non existing) its 20-the app also adds 2€ fee-you can also go to NMBS and have them arrange some-at no extra cost. You could also have bought the pass from them.

The app only gives an estimation of the reservation price, and without the booking fee that Interrail adds.

But also if you take that into account, these estimations are sometimes wrong.

Reservations can easily be made at Belgian stations that sell international reservations, without any added fees.