But we can debate as much as we like the differences, it is clear they are not normal ICEs other than domestically within Germany.
It was set up as a joint service concept but with two types of rolling stock. It does look like some of this has slipped, but it is not a normal ICE, and the correct reservation price is the EUR17 quoted by InterRail.
Just looking at ticket prices, and from a quick play with the DB and SNCF web sites looking at the 8:56 Frankfurt to Paris services.....
On DB, it
looks to me like any other DB long distance service, with Super Sparpreis, Sparpreis Europa and Flexpreis Europa fares available.
The Flexpreis is valid on any service on the chosen day,
but the trains are reservation compulsory.
SNCF offers prem's or leisure tickets, and also a Business Seconde ticket. The Business Seconde seems to be valid only for one particular train but with free exchanges up to 30 minutes after the train leaves.
The Flexpreis and Business Seconde tickets are both €138 though the prices for the cheaper tickets don't seem to agree (each company has their own quota?)
So the main difference from domestic DB long distance services - at least if you buy from DB - actually seems to be only the fact that it's compulsory reservation.
The reservations sold are - presumably - for someone with a Flexpreis ticket who wants to change the train they travel on and therefore a valid thing to sell.
As for whether you
should use it with an Interrail pass or not...
The Main in Seat 61 says the reservation fee is €18, quota controlled. I can't face looking at the Interrail web site and it probably won't tell me.
So I suspect (and do not insist because I am not an expert) that this reservation is
not supposed to be valid with an Interrail pass.
Maybe through a loophole it is actually valid. I don't know. This seems an unusual situation.
What would happen if you tried it I don't know either.
And finally, after all that I've just tried using the "Seat only (no ticket)" option to just make a reservation on the DB web site and it won't let me. Maybe I'm missing something.
Edit: I've just remembered that the original post was about a different route. But Strasbourg to Frankfurt seems to work the same way, including my inability to extract a reservation on its own. Though the Man in Seat 61 doesn't seem to give the price for an Interrail reservation on this route.