The title is very basic but I have a three-fold question about flight codes.
Who decides of a flight code, who uses it (or is expected to use it) and what does it mean?
We have a couple questions on the site about flight codes (usually called flight numbers). They deal with the fact that it can be flown by multiple airplanes, that it might (or might not) uniquely identify a set of origin, destination and departure time for flights or they deal with legality. I checked Wikipedia to learn more about it but while it seems to list what it is not (an airplane identifier) or list some traditions about numbering, it does not really define what it is (beside that it "identifies a flight", but I would think it defines a flight route, not just a flight).
So what does a flight code actually identify? A [commercial, passenger] flight route? Who decides/validates it? An airline, or the IATA, or some other organisation? And who is it intended for? Airports, airline employees as well as travelers?