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

    The item list XAB (XABITM) provides a convenient means for
    using item list information to support RMS functions. Each
    XABITM points to an item list that includes one or more entries
    representing either a set function or a sense function that can
    be passed to the application program by way of the RMS interface.

    Because the mode field in a XABITM can be used to either set
    or sense the items in the list, you cannot use a single XABITM
    to both set and sense a particular function. However, you may
    use multiple XABITMs, some for setting functions and other
    for sensing functions. RMS logically ignores items that are
    irrelevant to any particular function while acting on any item
    that is relevant.

    Each entry in the item list includes three longwords, and
    a longword 0 terminates the list. Also note that RMS does
    not validate the item list. If the item list is invalid, RMS
    indicates that the XABITM is not valid by returning the invalid
    XAB status (RMS$_XAB) in the RAB$L_STS field.

    You can store the item list anywhere within process readable
    address space, but any buffers required by the related function
    must be in read/write memory.

    The XABITM control block currently supports the following
    functions:

    o  Enhancements to network file access functions

    o  Passing of file user characteristic information

    o  Enhancements to RMS performance monitoring functions

    o  Support for compound documents

    o  Specifying the number of local buffers

    o  Expiration date and time suppression

    o  Support for file size in heterogeneous environments

    The following table lists the symbolic offset and a brief
    description of each field.

    Offset        Description

    XAB$B_BLN     Block length
    XAB$B_COD     Type code
    XAB$L_        Item list address
    ITEMLIST
    XAB$B_MODE    Set/sense control
    XAB$L_NXT     Next XAB address