The rules around flight numbers are, in simplified form, that the airline can use a flight number once per city pair per 24 hours. This allows reusing a flight

The rules around flight numbers are, in simplified form, that the airline can use a flight number once per city pair per 24 hours. This allows reusing a flight number -- possibly with a different plane -- on a later segment, which is a concern due to flight numbers being limited to four digits via legacy systems (airlines with lots of regional affiliate and partner codeshare flights literally risk running out of flight numbers!). In the US, larger airlines often use a flight number twice a day -- once for a domestic flight and once for an international flight. For flights departing to international destinations, it's not uncommon for one of the inbound feeder flights to use the same flight number. Also, for a foreign carrier, the rules are a bit more complex; they can make an additional stop in the US, and can pick up more passengers, but can't do much more than that. US federal law requires that transportation between two points in the US take place on a US-owned carrier. So, for example, Qantas flies from New York to destinations in Australia with a stop in Los Angeles, and can pick up more passengers at LAX and even change planes there for the onward journey, but they can't sell JFK-LAX tickets.