Stash is "just fabric." It can be used in any pattern. A UFO is already progressed into some form of an object - meaning a pattern has been selected, the fabric cut and the assembly has been started. At some point in the process, the project was put aside for whatever reason. Some UFOs are intended to be picked up again and put into the WIP (work in progress) state. Others will never be finished and linger in the UFO state until they are passed on.