The pass is your ticket. Reservations are required for some trains, such as TGVs in/to/from France. Domestic TGVs are €10 (limited availability), then €20 until they're full. International TGVs and ICEs to Germany are €17. Reservations within Germany are optional.

Pass holders who live in France can get reservations for the outbound and inbound journeys free of charge, but only at an SNCF ticket office. They're called "Trajet d’approche”. Possibly only for domestic reservations (within France). SNCF is not very forthcoming with information about it. (Edit: for the France-Germany pass, also for the TGV/ICE France-Germany, see below.)

Reservations cannot be booked in the rail planner app. The app can only forward to some of the websites where you can book, including the Interrail website. You can book reservations anywhere you like.

If you book on the Interrail website, you always pay an additional booking fee of €2 per person and train. And reservations in Germany are very expensive when you book them via Interrail: €6+2 = €8 instead of just €3 per train on tickets.oebb.at or €4.90 per journey (including all trains) on bahn.com.

If you live in France and use a regular Interrail pass, you get free domestic seat reservations.

If you live in France and use the special offer from the French & German governments, you get in addition free seat reservations for the international TGVs/ICEs. Have a look at : https://assistance.passefranceallemagne.fr/fr/support/solutions/articles/103000124118-comment-r%C3%A9server-gratuitement-le-trajet-aller-vers-l-allemagne-et-le-retour-vers-la-france-

They say on that link that there’s a dedicated page about that on SNCF website. There isn’t.

I guess you’d like to take the TGV between Lille and Strasbourg ? From there you can take the regional train to Appenweier/Offenburg or go via Lauterbourg. Also possible to go via Metz and Saarbrücken.

Wouldn't they just mean this page?

https://www.sncf.com/fr/offres-voyageurs/voyager-en-train/tgv-europe/passe-france-allemagne

It has some details about the free reservations on the ICE/TGV France-Germany.

Ah I guess then. I thought that it would be dedicated to the seat reservations.

Last time I looked at it, it didn't have the map. At least they update regularly. Although they could link it directly on passefranceallemagne.ft when they say "cette"/that page...

Hello ! Thank you so much for answering so fast at first ! 
 

i was trying to go from Lille to Berlin. The App Rail Planner your indicates me that there is a 22 euros from Lille to Brussels (as this is a TGV indeed). However I planned 2 other trains (ICE) with the French & German Governments pass. I actually have a QR code within the app. So I guess that works too right ?

1. The reservation fee Lille-Brussels is €20, plus €2 booking fee if you book via Interrail instead of via SNCF.  Regular tickets are often available for €19. But...
2. Brussels is in Belgium. Your pass won't be valid there.
3. The QR code in the app is your ticket and is therefore enough for any train that doesn't have mandatory reservations.
4. The rail planner app is not a reliable tool to plan. bahn.com or the DB Navigator app is a lot more reliable. DB Navigator is also good for keeping track of delays and the platforms you need to be at.

Once again, thank you so much.

One last thing. What trains does not need reservation in Germany ? 

I rearranged my trip to go from Paris-Est, to Frankfurt, then Berlin. 

I understand now that I have to pay 19 euros to go from Paris to Frankfurt. 

Do I have to pay something else from Frankfurt ? Or is the ticket enough , 

You better ask the other way around: which train need a reservation? Night trains. Long distance day trains (ICE, IC etc.) within Germany have optional reservations.

The reservation is €17, but you can get it for free at the SNCF ticket office, as @thibcabe showed above.

You don't have to book a reservation, it's optional.

Alright so no reservations needed !

Thank you so much for all the replies and the quickness that came with it. 

No reservations needed within Germany indeed, you should always find 2 available seats (go to train ends as people are lazy and board in the middle).

However it is needed for Paris - Germany. Book well in advance at a SNCF ticket counter, those trains are extremely popular due to summer + your special pass. When would you like to travel ?

The direct TGV between Lille-Europe and Strasbourg (avoiding the Paris transfer) runs at 08:02. (The 14:03 TGV runs too late to reach Berlin)

I will go directly to Paris-Est by car.

The trip I want to do is Paris-Est → Frankfurt at 7:23 am

and then go from Frankfurt to Berlin directly ! At the 18th of July 

Ah yes that's totally fine :)

There are only 7 seats left on the 07:20 train so book ASAP.

Enjoy your trip !

Thank you so much !