INSTANCE_GUID
InstallShield 2026 » InstallScript Language Reference
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This system variable contains a globally unique identifier (GUID) for the installation, which is used as the name of the application uninstallation registry key. This variable's value is set at run time to equal PRODUCT_GUID except in a multi-instance installation.
The value of this system variable is shared among object scripts and between object scripts and the main installation script. You cannot assign a new value to this system variable.