Flight code

A flight number, when combined with the name of the airline and the date, identifies a particular flight. This callsign should not be confused with the tail number of the aircraft, although both can be used as a call-sign as used in general aviation. A particular aircraft may fly several different flights in one day, and different aircraft may be used for the same flight number on successive days.

A number of conventions have been developed for defining flight numbers, although these vary widely from airline to airline. Eastbound and northbound flights are traditionally assigned even numbers, while westbound and southbound flights have odd numbers. Other airlines will use an odd number for an outbound flight and use the next even number for the reverse inbound flight. For destinations served by multiple flights per day, numbers tend to increase during the day. Hence, a flight from point A to point B might be flight 101 and the return flight from B to A would be 102, while the next pair of flights on the same route would usually be assigned codes 103 and 104.

Flight numbers of less than three digits are often assigned to long-haul or otherwise premium flights. Flight number 1 is often used for an airline's "flagship" service. For example, British Airways flight 1 was the early morning supersonic Concorde service from London to New York City and is now a premium business-class only flight between the same cities; Air New Zealand flight 1 is the daily service from London to Auckland via Los Angeles; Qantas flight 1 is the daily Kangaroo Route from Sydney via Dubai to London. American Airlines Flight 1 is the daily flight from New York to Los Angeles; and El Al flight 1 is the daily overnight service from Tel Aviv to New York City. Four-digit numbers in the range 3000 to 5999 typically represent regional affiliate flights, while numbers larger than 6000 are generally codeshare numbers for flights operated by different airlines or even railways.

...
Wikipedia