A character string containing the system message that corresponds
    to the argument you specify.

    Note that, although each message in the system message file has
    a numeric value or range of values associated with it, there
    are many possible numeric values that do not have corresponding
    messages. If you specify an argument that has no corresponding
    message, the F$MESSAGE function returns a string containing the
    NOMSG error message.

    For more information on system error messages, see the OpenVMS
    System Messages: Companion Guide for Help Message Users.