This is typically done when a flight is 'stubbed'. For example, ASQ931 (DL4931) is ROC
. Often the ROC
leg is delayed (like today!). If an extra ship and crew can be found, the second leg will be stubbed, meaning they'll grab the new ship and cew and use them. This is usually only done when the first leg is extremly late. Because of this, what often happens is that both legs of the flight can be in the air at the same time, since the first leg is delayed, and the second would now be on time. But no flight number can be used twice at the same time, so the second flight is amended with A, but only to ATC and the pilots. So they would use ASQ931A, pronounced "Acey 931 Alpha". I have seen B used as well.
Note, this is usually only done if there has not been a set range of flight numbers set aside for stubbed flights. For example, Delta Mainline uses some 9000 numbers for stubbed flights.