The AdBlue tank (DEF tank) stores the urea solution needed for emissions control. Made of heavy plastic, it is separate from the diesel fuel tank. It features a blue filler cap, usually located next to the diesel fuel neck, under the hood, or in the trunk. The tank includes a heater element to melt frozen fluid in winter, a level sensor to track fluid levels, and a temp sensor to protect the system.
Never put diesel fuel into the AdBlue tank, or AdBlue into the diesel tank. Even a small drop of diesel will destroy the rubber seals in the pump and injector, requiring a complete system replacement. If you accidentally mix them, do not start the engine. You must tow the vehicle to a shop and drain the tank completely.