Depends on the airport. But it can be as simple as ORD says "we need to everyone to cut 20% of their flights arriving and departing." For DL 20% might only be one flight, and they chose a flight from DTW (for example) to delay or cancel instead of a flight from ATL. For UA, that 20% a lot more flights, and it's not as simple as choosing the destination, but rather looking at the network as a whole. Where do they need the plane and/or crew next? How many passengers are affected? Priority may be given to long hauls flights and/or widebodies to once a day destinations vs. a domestic flight with alternatives to get there.