/sys$common/syshlp/helplib.hlb Sys Parameters, ACP_EXTLIMIT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    ACP_EXTLIMIT specifies the maximum amount of free space to
    which the extent cache can point, expressed in thousandths of
    the currently available free blocks on the disk. For example,
    if available free space on the disk is 20,000 blocks, a
    specification of 10 limits the extent cache to 200 blocks.

    The computed, installed value is usually adequate. Users with
    four or more OpenVMS Cluster node systems might want to adjust
    this parameter.

    ACP_EXTLIMIT is a DYNAMIC parameter.