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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?

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The flight number has basically two parts, the first two characters identify the airline selling the flight and the remaining digits are the airline's flight identifier.

The first two digits are the IATA assigned airline code, which were originally based on the airlines name, ie: BA - British Airways, DL - Delta, etc. But due to the proliferation of airlines, many newer carriers are simply assigned whatever 2 characters are available still (including numerals, which are usually paired with one letter).

The remaining characters (anything from 1 to 4) are the airlines internal designator for that flight. Most airlines have a numbering system, such as 1 to 1999 for domestic flights, 2000-4999 for international flights, 5000 to 7999 for code share flights, etc. But there is no standard, each airline creates their own.

Likewise the airlines are free to choose how to assign flight numbers to a flight or series of flights. This aspect can be more confusing, because there is not much consistency even within a single carrier. A flight number can be assigned to a flight that just goes from A to B. A flight number can be assigned to a flight that goes from A to B to C (even one that changes aircraft at B). A flight number can be assigned to a round trip going from A to B to A.

Code share flight numbers can sometimes match the operating airline's number or more often simply use the selling airline's numbering system.

The flight number used for air traffic control is different, as the airline portion is a three letter code combined with the original flight number. They also use the tail number, which is unique for each individual aircraft. Tail numbers are assigned based on rules of the country in which the aircraft is registered.

Because of the numerous airlines cropping up, the three digit airline codes could soon be commonplace on the travelers side of the flight as well.