The installer sets this variable to the drive letter and colon (or network share, for shared Windows installations) that contains the Windows folder on the target system.
Note: Prior to InstallMate 7, the installer set this variable to the default drive for the installation, which is the local disk volume with the most free disk space. From InstallMate 7 onwards, ROOTDRIVE is the same volume as WindowsVolume.
You can refer to it as <ROOTDRIVE>. Set automatically by the installer.