What is the difference between a Unity .STU project and an .XEF project?

Issue:

What is the difference between a Unity .STU project and an .XEF project?

Product Line:

Unity

Resolution:

An .STU file is a project created in Unity Pro.
An .XEF project is a Unity Pro application exchange file. It is a project that has been exported from Unity Pro.
NOTE: Any file extension beginning with an 'X' indicates the XML format (extensible markup language).