VMS Help FORTRAN, Logical Names, FORnnn  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  The file specification 'FORnnn', where 'nnn' is the logical unit
  number, is used as the file specification when a logical unit is
  opened and the FILE= keyword was not specified in an OPEN
  statement.  Similarly, 'FORnnn.DAT' is used as the default file
  specification unless the DEFAULTFILE= keyword was specified.
  FORnnn may be defined as a logical name, in which case it is used
  by OpenVMS RMS to specify the file to be opened.