DEF, short for diesel exhaust fluid, is a clear liquid made of urea and purified water that modern diesel engines use to cut the pollution coming out the tailpipe. It is not a fuel and goes in its own separate tank.
Today’s diesel trucks and equipment run a cleanup system that sprays a fine mist of DEF into the hot exhaust. There the urea, a compound also used in fertilizer, breaks down and turns harmful nitrogen oxides, a major part of diesel pollution, into plain nitrogen and water. This setup is called SCR, for selective catalytic reduction, and the law requires it on modern diesels.
DEF burns alongside diesel in the sense that a truck uses it up as it drives, roughly a couple of gallons of DEF for every hundred gallons of diesel. It goes in a separate blue-capped tank, and if that tank runs dry the engine is built to slow down or refuse to run, so a fleet cannot simply skip it.
For a jobber or marketer, DEF is a natural side product. The same fleets buying diesel need DEF on a steady schedule, and it sells by the jug, the tote, or in bulk into a tank. It carries a healthier margin than fuel and rides along on deliveries the company is already making, so it adds revenue without much added cost.
In useThe jobber drops diesel at the trucking fleet and tops off the DEF tank on the same stop, selling it in bulk at a better margin than the fuel itself.
See also Dyed diesel, Jobber