Diesel exhaust fluid is a liquid used to reduce the amount of air pollution created by a diesel engine. DEF is not added to diesel engines. but it is injected in the exhaust stream. It is a non-hazardous solution of 32.5% urea in 67.5% de-ionized water. DEF look alike same water and clear colorless.

Yes, Diesel Exhaust Fluid is required, because since 2010 are all equipped with SCR and that reason DEF is Required for passenger vehicles and trucks.

Diesel Exhaust Fluid is required Passenger vehicle (cars and vans), truck and bus.

When temperature drop below 12 degrees Fahrenheit that time DEF is freeze

It's depended on DEF tank. If your DEF tank 1 gallon of then refill every 300 - 500 miles.

The American Petroleum Institute recommends that you determine when DEF was put in the tank. The storage life of DEF is maximum 12 month in ultra-condition. If DEF has been stored in the vehicle more than 12 months, it is recommended that it be replaced.

Urea and consequently DEF is not overly toxic to human. It can be irritating to your eyes and skin but not life menacing. High concentrations in the blood can be harmful to humans, however ingestion of low concentrations of urea, due to drinking enough water, is not harmful.