VMS Help CMS, RESERVE, Qualifiers, /NOCONCURRENT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Specifies that the element cannot be reserved by another user
    while you have it reserved. You must replace or unreserve the
    element before others can reserve it. By default, CMS allows
    concurrent reservations if the element has the concurrent
    attribute set.