Here is DEF pump from current gen Ford SuperDuty trucks (aka “reductant pump” in Ford terminology). DEF stands for Diesel Exhaust Fluid. Also known as “urea” which is not quite correct as DEF is urea in water solution.
The whole system known as SCR and consists of separate DEF tank installed under the frame, usually near the fuel tank (but could be moved to a different spot if truck is modified for some purpose). Inside the tank installed sender unit that has a small filter on intake, short pipe with built-in heating element and level sensor with three electrodes (so the system control unit can only detect if tank is 100% full, 50% full or about empty). On top of the sender unit secured this DEF pump assembly with inlet on the bottom and outlet to the side (right side on this picture). This assembly has actual pump driven by variable speed motor with its control circuitry (on the back), solenoid valve that directs fluid flow to the feed line or back to the tank (on the left side), pressure sensor (on the front) and another heater (two pins on the back side connector). One more heater, the third one in the system, is built into the feed line along its whole length. So there is a bunch of wires coming to the DEF pump and tank but it is easy to figure out what every wire is for.
Then there is another part of the system fitted to huge exhaust pipe with catalytic converters and soot filter. Here is that pipe laying on the floor
Right after the primary cat there is a simple electromagnetic injector for DEF. Two NOx sensors, that are kind of modified wideband oxygen sensors installed there: one before NOx cat and another after it so the system can see the content of NOx and efficiency of the NOx cat (and whole system) operation. Each NOx sensor has its own control unit that supplies converted NOx sensor signal to PCM. Some variations of SCR system do not use secondary NOx sensor. All three heaters in the system also controlled by separate control unit – glow plug module. And PCM controls and monitors the rest (including four exhaust gas temperature sensors fitted to exhaust pipe).
Altogether we have three or four different control units to control this system designed for a sole purpose to reduce nitrogen oxides emissions even further than before.