Allegiant Air , JetBlue , and Southwest Airlines are some US-based airlines that may use their focus cities as an airline hub .
However , with the term 's expanded usage , a focus city may also function as a small-scale or total hub , Allegiant Air , JetBlue and Southwest Airlines are examples of US-based airlines that consider some of their focus cities run like a hub , but Delta Air Lines is one example that does not use the same requirements as the previous three.
Airline hub