At all of the places I have used the card at, the TDS card is your "credit card" that is used at the pump. Basically, swipe card, enter pin, select if you want to purchase DEF (not discounted) after filling up with diesel. Complete the fill and hang the pump handle up. That's pretty much it unless you selected DEF and you would then fill up your DEF tank with the other nozzle. As for the discounts, the amount varies at each location. You have an app "OpenRoads" that will show you the locations and amount of discount at each location as well as any transaction fee (usually around $0.40) that might apply. So far our discounts have ranged around $0.20-$0.60 per gallon. There is no cost to use the program so we see it as a win-win. The stores we have used the card are are mostly LOVES, Allsups, 7-11 truck centers, Stripes, etc. There are no Flying-J or Pilots in the program so we use our GoodSams discount at those locations.
Here is the link to their FAQ page: MyOpenRoads FAQ Page