OK, got some clarification from my SIL. Her husband's personal truck uses the DEF stuff. It's a pollution control additive that gets injected into the exhaust system to reduce emissions. (His truck tells him when he needs to add some usually a gallon or two). It doesn't get mixed with the diesel. It goes in a separate tank in his truck, and he gets it from a separate nozzle at certain truck stops. Like 72% de-ionized water and the rest is urea.
Apparently there's alot of people trying to find a work-around for it, as having your truck or tractor be idle because you can't reduce emissions sounds jack.