Controls whether CMS maintains a reference copy of the element.
    You must have established a reference copy directory.

    The presence of the reference copy attribute for an element is
    inherited from the library, that is, if a reference copy directory
    is established for the library, by default, the attribute is
    enabled for the element. You can override the reference copy
    attribute by specifying /NOREFERENCE_COPY.

    If a reference copy directory has been established for the CMS
    library, CMS creates a reference copy of the new element and
    updates the reference copy directory each time you create a new
    main line generation of that element. When CMS places a file in
    the reference copy directory, it also deletes any earlier versions
    of that file in the reference copy directory.