The driver/operator of the vehicle fills Diesel Exhaust Fluid into a dedicated Diesel Exhaust Fluid tank (which is separate to the diesel tank) located on the vehicle chassis. There is usually a blue filler cap/lid which states "Diesel Exhaust Fluid" on the top. Diesel Exhaust Fluid is automatically injected into the tail pipe of the vehicle from the Diesel Exhaust Fluid tank by the SCR technology.This is before the SCR catalyst, but after the engine.
As the Diesel Exhaust Fluid heats up in the exhaust gases, it converts into CO2 and ammonia. Once inside the SCR catalyst, the nitrogen oxide (NOx) gases from the exhaust pipe, react with the ammonia. This changes the harmful NOx gases in the exhaust to nitrogen and steam which is released to the atmosphere. Nitrogen is an already occurring harmless atmosphere gas.
Diesel Exhaust Fluid is usually replenished during your service or sometimes your Diesel Exhaust Fluid warning light will indicate that you need to refill beforehand. Running dry of Diesel Exhaust Fluid may cause your vehicle to operate in crawler mode (go slow). Refilling is easy, use clean Diesel Exhaust Fluid apparatus (ie don't use oil or diesel funnels), take off the filler cap, pour until full. For smaller vehicles the tank cap is usually in the boot, under the carpet. Wipe up any spills as it can corrode some metals/paintwork.
Diesel Exhaust Fluid is ideally purchased from your Car/RV/Pickup truck dealership. However, it is also available at Truck stops, truck dealerships and Oil distributors who supply the truck industry. They may have Diesel Exhaust Fluid containers or pumps that passenger vehicles can use, but please investigate on arrival.