/sys$common/syshlp/helplib.hlb Sys Parameters, POWEROFF  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    POWEROFF enables or disables software requests to the console
    firmware to remove power from the system. This parameter should
    normally be turned ON (1)  to allow software to make power-off
    requests. However, POWEROFF can be set to OFF (0)  to disable
    software power-off requests.

    If firmware or hardware support for the power-off request is not
    implemented, the shut-down procedure will leave the system halted
    but fully powered.

    POWEROFF is a DYNAMIC parameter.