14.8.2 BUFFER Clause
  The BUFFER clause applies to disk files of any organiza-
  tion. In the case of sequential files, the BUFFER clause sets
  the number of blocks read in on each disk access. For rel-
  ative and indexed files, the BUFFER clause determines the
  number of I/O buffers that are allocated. In general, the
  OpenVMS operating system supplies adequate defaults for all
  file types; therefore, the BUFFER clause is rarely necessary.

  You can specify up to 127 buffers as either a positive or a
  negative number:

  .
        If (0 < BUFFER < 127), RMS allocates enough space for
        the specified number of buckets.
  .
        If (-128 < BUFFER < 0), BASIC allocates the absolute
        value of the specified number of buffers.
  .
        If (BUFFER=0), BASIC allocates the process default for
        the particular file organization and device-this value is
        usually adequate.