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

    Specifies whether elements in a library can have multiple
    reservations by setting the concurrent attribute on the library.
    Use /NOCONCURRENT to disallow multiple reservations. If this
    qualifier is not specified the status of this attribute remains
    unchanged.