soemthing that you are to stupid and minor to unsderstand
Definition is the brief explanation of anything.
A system is a group of independent but interrelated elements comprising a unified whole.
Yes, actually most comes from the outside, the rest comes from within the aircraft.
The English phrase 'ventilation system' means a mechanical system in a building that provides fresh air
More information is needed but generaly its an emissions system to inject fresh air into the exhaust system to reduce emissions..............