This variable is the alias for the HKEY_USERS registry root and is used to abbreviate the registry hive name HKEY_USERS in registry paths.
Used internally by InstallMate. You can refer to this variable as <HKU>. It is normally used as part of a registry path, as in <@<HKU>\...remainder of path...>. This variable is read-only; do not try to change it.