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

  The 'sharing' is a value that specifies whether  other  programs
  can  access  the  file  while  it  is open.  A value of READONLY
  indicates that other programs can read  but  not  write  to  the
  file.   This  is  the  default  value  for files with HISTORY :=
  READONLY.  READWRITE indicates that other programs can read  and
  write  to  the file when it is open.  A value of NONE denies any
  access to the file while it is open.  This is the default  value
  for all other histories.