The installer sets this variable to the Windows version number of the target system if the target system is a 64-bit Windows NT version. Otherwise it is not set (or set to 0). The Windows version number is encoded as 100 * major + minor.
For a full list of VersionNT64 values, see the documentation of the VersionNT variable.
You can refer to it as <VersionNT64>. In conditional expressions you can omit the brackets and use tests like VersionNT64 >= 502. Do not set this variable directly; it is set automatically when the installer runs.