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

    The record access block (RAB) defines run-time options for a
    record stream and for individual operations within a predefined
    record stream context. After you connect the file to a record
    stream and associate the record stream with a FAB, you use the
    RAB fields to specify the next record you want to access and to
    identify appropriate record characteristics.

    NOTE: If you are using 64-bit addressing on an OpenVMS Alpha
    system, you must use RAB64 instead of RAB.

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

    Offset      Size   FDL Equivalent           Description

    RAB$B_BID   1      None                     Block identifier
    RAB$L_BKT   4      CONNECT BUCKET_CODE      Bucket code
    RAB$B_BLN   1      None                     Block length
    RAB$L_CTX   4      CONNECT CONTEXT          User context
    RAB$L_FAB   4      None                     File access block
                                                address
    RAB$W_ISI   2      None                     Internal stream
                                                identifier
    RAB$L_KBF   4      None                     Key buffer address
    RAB$B_KRF   1      CONNECT KEY_OF_          Key of reference
                       REFERENCE
    RAB$B_KSZ   1      None                     Key size
    RAB$B_MBC   1      CONNECT MULTIBLOCK_      Multiblock count
                       COUNT
    RAB$B_MBF   1      CONNECT MULTIBUFFER_     Multibuffer count
                       COUNT
    RAB$L_PBF   4      None                     Prompt buffer address
    RAB$B_PSZ   1      None                     Prompt buffer size
    RAB$B_RAC   1      CONNECT                  Record access mode
    RAB$L_RBF   4      None                     Record buffer address
    RAB$W_RFA   6      None                     Record file address
    RAB$L_RHB   4      None                     Record header buffer
                                                address
    RAB$L_ROP   4      CONNECT                  Record-processing
                                                options
    RAB$W_RSZ   2      None                     Record size
    RAB$L_STS   4      None                     Completion status
                                                code
    RAB$L_STV   4      None                     Status value
    RAB$W_STV0  2      None                     Low-order word status
                                                value
    RAB$W_STV2  2      None                     High-order word
                                                status value
    RAB$B_TMO   1      CONNECT TIMEOUT_PERIOD   Timeout period
    RAB$L_UBF   4      None                     User record buffer
                                                address
    RAB$W_USZ   2      None                     User record buffer
                                                size
    RAB$L_XAB   4      None                     Next XAB address