VMS Help CMS, MODIFY, LIBRARY, Qualifiers, /REFERENCE_COPY=directory-specification  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Specifies a valid VMS directory to be used for reference copies
    of library elements. The directory cannot be a CMS library, nor
    can it be a subdirectory of a CMS library directory. If you use
    the MODIFY LIBRARY command on a search list consisting of more
    than one library, you should specify a reference copy directory
    for each library in the search list. If you specify only one
    reference copy directory for more than one library, CMS uses one
    reference copy directory for the entire search list, not one
    reference copy directory for each library in the search list.

    Use the /NOREFERENCE_COPY qualifier to remove the connection
    between the current CMS library and the current reference copy
    directory. Wildcards are not allowed.