The installer sets this variable to a nonzero value if the installer runs from an existing installation on the target system; it is set to 0 if the installer runs from its distribution package. You can use this variable to distinguish between a maintenance installation or uninstallation (both of which run from the existing installation) and a new installation or upgrade (which run from the distribution package).
You can refer to this variable as <TsuInstalled>. In conditional expressions you can omit the brackets and simply use TsuInstalled as a test for a maintenance installation as opposed to an installation from the distribution media. Set automatically by the installer.
Tip: To check if the product is already present on the system before the installation starts (as opposed to testing if the installer runs from an existing installation), check the InstalledVersion variable. This variable is empty if the product is absent, and non-empty if present. The conditional expression InstalledVersion is True if the product is already present on the target system; conversely, NOT InstalledVersion is True if not.