The GUID is he unique identifier of the extension. Two extensions with the same GUID will be considered by ACT as the same extension.
This is very important when the extension is deployed to ensure that two different extensions will never be in conflict (have the same name, for example).
This identifier must be added at least before the first build of the extension and must never changed after that. When the extension is updated, a new version created, or features added... the GUID must be kept unchanged.
Examples
A GUID can be generated from the project's page command window using the following code: