For me, a WIP is something you're actually working on.
A UFO is something you've set aside, unfinished at whatever stage, and are not currently working on.
Say you're working on one thing, and for whatever reason, you start something else. If you continue to work on both, they're both WIPs. But if you work only on the second one, even if you intend to finish the first one just as soon as you're done with the new one, the first one is a UFO until you start working on it again.
Granted, you could assign a minimum amount of time before it becomes a UFO. If you stop to make a one-day or weekend project, then for that couple of days it would be silly to think of the other project as a UFO.