VMS Help PASCAL, Input Output, General, OPEN, Organization  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  The  'organization'  is  a  value  that   specifies   the   file
  organization.   If  you  are  accessing  an  existing  file, the
  specified  organization  must  match  the  organization  of  the
  existing file; if it does not, an error occurs.  The choices for
  this parameter  are  SEQUENTIAL,  RELATIVE,  and  INDEXED.   The
  default is SEQUENTIAL.  The parameter choices are as follows:

   o  SEQUENTIAL file organization specifies that file  components
      are  stored  one after the other, in the order in which they
      were entered into the file.   Compaq  Pascal  supports  this
      organization   for   files   on  disk.   This  is  the  only
      organization  supported  for  files  on  magnetic  tape,  on
      terminals, on card readers, and on line printers.

   o  RELATIVE  file  organization  consists  of   a   series   of
      fixed-length    component    positions    (cells)   numbered
      consecutively from 1 to n.  The numbered, fixed-length cells
      enable  Compaq  Pascal to calculate the component's physical
      position in the file.  The cell numbers are called  relative
      component numbers.  Compaq Pascal supports this organization
      on disk files only.

   o  INDEXED file organization specifies that, in addition to the
      stored  components,  there exists at least a primary key and
      possibly  alternate  keys  (first  alternate   key,   second
      alternate  key,  and  so  forth).   Compaq  Pascal  uses the
      primary key to store components and uses a program-specified
      key  or  keys to retrieve data.  Compaq Pascal supports this
      organization on disk files only.