Most airlines use some variant of this system.

• Flight numbers can never be more than four digits, due to computer numbering restrictions.
• Super low flight numbers (like between 1 and 10) are usually for an airline's "flagship" flights. For instance, on Alaska, flight 1 connects Seattle and Washington, DC. For United, flight 1 connects San Francisco and Singapore. For Delta, flight 1 connects New York and London. For Southwest, flight 1 connects Dallas and Houston.
• Numbers between 100 and 999 are usually for an airline's longer or busier routes.
• Numbers between 1000 and 2999 are usually for an airline's shorter routes, but those are often still operated by the airline directly rather than regionals. (As an example, Southwest 1528 goes from Baltimore to Boston).
• Numbers between 3000 and 7999 or so are usually for regional flights. Southwest does not have any regional carriers, but for instance, a Delta flight with these numbers may actually use Endeavor, a United flight may use Republic, and an Alaska flight with these numbers may actually use Horizon.
• Numbers between 8000 and 9999 are usually for repositioning ("ferry") flights, maintenance flights, or charter flights. It is generally not possible to purchase a ticket for these.
• Flight numbers often increase throughout the day for the same route, but many airlines have moved away from this due to the increase in flights and resulting lack of number availability.
• Flight numbers may also indicate direction, with even numbers indicating eastbound and northbound flights, while odd numbers indicate westbound and southbound ones. But again, some airlines have moved away from this due to lack of number availability.