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

    Identifies a valid CMS library that is the location of the class
    specified by the old-class-expression parameter. When you
    specify an alternative library, the new-class-name parameter is
    optional. If you do not specify a value for /LIBRARY, the current
    CMS library is used.