VMS Help CMS, CREATE, 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.
    Use MODIFY LIBRARY to change this attribute on an existing library.