VMS Help Sys Parameters, XQPCTL2  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    XQPCTL2 controls improved concurrency. The default value of
    XQPCTL2 is 1, which turns on improved concurrency. Setting
    XQPCTL2 to 0 turns off improved concurrency. This parameter
    affects local access to the extent and file ID caches.

    This special parameter is used by Compaq and is subject to
    change. Do not change this parameter unless Compaq recommends
    that you do so.