The installer sets this variable to the Windows version number of the target system if the target system is running Windows 95, 98, or Me. Otherwise it is not set (or set to 0). The Windows version number is encoded as 100 * major + minor.
For a full list of Version9x values, see the documentation of the VersionNT variable.
You can refer to it as <Version9X>. In conditional expressions you can omit the brackets and use tests like Version9X >= 410. Do not set this variable directly; it is set automatically when the installer runs.