We talk of code sharing when two or more airlines share a scheduled service. Each of the airlines involved offers the flight concerned under its own flight number, but it is operated by only one airlines. So there can be different flight numbers for a single flight.

One example from the DUS timetable

Delta Air Lines, KLM  and Air France offer a nonstop flight to Atlanta (ATL) from Düsseldorf on a code sharing basis. The KLM flight number for this flight is KL 6025, the Air France flight number is AF 3614 and the Delta Air Lines flight number is DL 091. The flight, however, is operated by Delta Air Lines.

Your ticket shows the flight number of the airline that issued it. On the information boards in the terminal, in the app and on the websites, all flight numbers associated with the flight are shown on a rolling basis with the notation “Code Sharing”.

Airline Alliances

The code sharing method is mainly used within airline alliances like Star Alliance, One World or Skyteam. This allows the airlines to offer flights that they do not operate themselves, but are operated by a partner airline of the alliance concerned.