Originally Posted by orca15

To explain SDC even a little more, it is the process of calling within 24 hours of a flight and changing to another flight that leaves the same day. Your change is confirmed when you call, as opposed to being standby at the gate, so you know you have a seat. There are fare class restrictions for economy fares, but F fares are more flexible.

That was in ATL's response, but I thought a general description might help.