In InstallScript, the MAINTENANCE system variable is set to True when an installation is being run in maintenance mode. This article explains how this variable is set to True.
The MAINTENANCE property is set upon the existence of the uninstall log that is created by the InstallScript section of the project. If this log exists, the property is set to True.
The way that InstallShield determines whether or not there is a log file is by looking in the registry. In the registry, InstallShield looks in the HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Uninstall\ProductCode key for the path to the log file. If the key does not exist in this location, InstallShield looks in the HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Uninstall\InstallShield Uninstall Information location. The key should exist in one of these locations and upon finding it, the MAINTENANCE system variable is set to True.