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

    The summary XAB (XABSUM) can be associated with a FAB at the time
    a Create, Open, or Display service is invoked. The presence of
    this XAB during these calls allows RMS to return to your program
    the total number of keys and allocation areas defined and the
    version number when the file was created. Note that a XABSUM is
    used only with indexed files.

    The following table lists the symbolic offset, the size in bytes,
    and a brief description of each field.

    Offset      Size     Description

    XAB$B_BLN   Byte     Block length
    XAB$B_COD   Byte     Type code
    XAB$B_NOA   Byte     Number of allocation areas defined for the
                         file
    XAB$B_NOK   Byte     Numbers of keys defined for the file
    XAB$L_NXT   Longword Next XAB address
    XAB$W_PVN   Word     Prolog version number