ProExtensionUUID
A UUID string that uniquely identifies your extension to the Compressor app.
Discussion
The value for this key is a placeholder UUID the Xcode template generates. Each extension must have a unique UUID. When you build an extension for the first time, the build script in the Xcode template replaces the placeholder UUID with a new UUID. The new UUID fulfills the uniqueness and persistence requirement for ProExtensionUUID. For subsequent rebuilds, the UUID stays the same because the Compressor app uses this UUID to differentiate between previously saved and newly discovered extensions.