It is squirted into the exhaust to chemically clean it up before exiting. If you run the tank empty, trucks that use it now go into limp home mode. The O'Reillys rep was telling me about it this morning. He said that most medium duty and heavy trucks currently using it consume about 2 gallons of the urea **** for every 100 gallons of fuel. Not too bad price wise but remembering to top it off would probably be a PITA.