i used them a couple of times last year ,one was Arras where you can't book and the other was Dijon which was outside the campsite Camping Du Lac Kir which again you didn't have to prebook and that had been taken back over by the campsite. I have signed up on their website and if you pre book they will send you a qr code to print off for access to their sites which is how you gain entry at Arras but you pay by card and it spits a load of qr codes at you, so a little different to campingcarparks where you load the card for entry.
Ps , you don't need to prebook you can pay by card at the entry where it will issue the codes for the electric ,water and entry and exit , beware if they are all like Arras the tickets spew out really fast i you have to be quick to catch them which could be a problem if its windy