If you take a look in the Registry Editor and navigate to HKEY_CURRENT_USER\Software\Bentley\InRoadsGroup\Applications, you will see the folders for the different installed versions. In the example below, I have XM, SS2 and SS3 installed:
Version 8.11 is the SS2 version.
Version 8.11.9 is the SS3 version.
Version 8.9 is the XM version.
Each of these folders contain their own Project Defaults key.