Code-share and interline agreements
Each airline has its own two-letter code, which was assigned by IATA. For examples, OS for Austrian Airlines, KL for KLM, LH for Lufthansa and etc.
Some airlines have an agreement under which they have the right to perform the same flight in one day and at the same time. What does it mean?
Searching for flight on Tripway.com you can find the same flight but from two different airlines (British Airways and Austrian Airlines for example) and with different prices. It is possible when your flight performed by British Airways, by British Airways plane, with the crew from the British Airways, but tickets can sell two airlines (British Airways and Austrian Airlines). In other words, one of these airlines allows another carrier to sell its own flights and issue tickets.
Note, Code-share flights have four numbers after airline code (OS 4490) and others have only three (BA 576).
Remember, when you buy a ticket on a code-share flight that will be served by British Airways partner airline (for example), then the rules of this airline will be apply.