14.4 I/O and Record Buffers
  An I/O buffer is a storage area in your program that RMS
  uses to store data for I/O operations. You do not have direct
  access to I/O buffers; they are controlled entirely by RMS.
  The I/O buffer holds blocks of data transferred from the de-
  vice, and its size is always greater than or equal to that of
  the record buffer. For more information about the amount
  of storage allocated for I/O buffers, see the OpenVMS Record
  Management Services Reference Manual .

  A record buffer is another storage area in your program.
  You have direct access to and control of the record buffer.
  When your program reads a record from a file, the informa-
  tion is transferred from the file to the I/O buffer in one large
  chunk of data, and then the requested record is transferred
  to the record buffer. When your program writes a record,
  data is transferred from the record buffer to the I/O buffer,
  and then to the file either when the I/O buffer is full or when
  other blocks need to be read in.

  You can use MAP statements to create static record
  buffers and associate program variables with areas (fields)
  of the buffer. Static record buffers are buffers whose size does
  not change during program execution and whose program
  variables are always associated with the same fields in the
  buffer.

  You can create dynamic record buffers with either a MAP
  DYNAMIC or a REMAP statement. These statements, when
  used after a MAP statement, associate or reassociate a par-
  ticular program variable with a different area (field) of the
  record buffer. However, the total size of a record buffer does
  not change during program execution.