I discovered by accident that there is a price difference for a seat reservation with interrail!
Via a website of B europe (photo) you will find prices of 10 euros for a connection with 2 TGV for example Lille - Marseille or Lille - Bordeaux!
This is always with a change in Paris.
The interrail website charges 20 euros for this! Both without the mandatory reservation costs.
Could this be an error on the B europe website? Otherwise this is very interesting.