InstallMate sets this variable to the version number of of the main product, if the current installer is an add-on or upgrade installer as defined by the Add-on/Upgrade for product attribute on the Product info project page. If the current installer is not an add-on or upgrade installer, then its value is unknown.
You can refer to this variable as <MainInstalledVersion>. In conditional expressions you can omit the brackets and simply use MainInstalledVersion as a test for main product installation. Do not change its value; it has been carefully configured for cross-platform compatibility.