Diesel Exhaust Fluid (DEF) is a liquid mix of approximately one-third ammonia (Urea) and de-ionized water. It weighs more than water, and the tanks take up valuable frame space. It can be purchased nearly everywhere, so no need to over spec the volume on a new truck. The fluid is sensitive to heat, being that it contains a high percentage of water. Cold climates do not bother it, engines have a warm-up period where dosing is not required early in the operational cycle, allowing for thaw.