Library /sys$common/syshlp/lse$clihelp.hlb SET, DEFAULT_DIRECTORY, Parameters  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 device-name[:]
    Specifies a device name to be used as the default device in a file
    specification.

 directory-spec
    Specifies a directory name to be used as the default directory
    in a file specification. A directory name must be enclosed in
    brackets. Use the minus sign to specify the next higher directory
    from the current default directory.

    You must specify either the device-name parameter or the
    directory-spec parameter. If you specify only the device name, the
    current directory is the default for the directory-spec parameter.
    If you specify only the directory name, the current device is the
    default for the device-name parameter.

    You can use a logical name, but it must constitute at least the
    device part of the specification.