Setting this property is like setting the TransformsAtSource policy policy except that the scope is different. Setting TransformsAtSource policy applies to all packages installed by a given user. Setting the TRANSFORMSATSOURCE property applies to the package regardless of the users.
Windows Installer interprets the TRANSFORMSATSOURCE property as though it were the TRANSFORMSSECURE property. If the @ flag is passed in the TRANSFORMS property, Windows Installer treats the transforms in the list as secure-at-source transforms.
Windows Installer 5.0 on Windows Server 2012, Windows 8, Windows Server 2008 R2 or Windows 7. Windows Installer 4.0 or Windows Installer 4.5 on Windows Server 2008 or Windows Vista. Windows Installer on Windows Server 2003 or Windows XP. See the Windows Installer Run-Time Requirements for information about the minimum Windows service pack that is required by a Windows Installer version.