Indicates the logical unit to which a file is to be connected.  It
  takes the following form:

  [UNIT=] u

  u  Is a numeric expression

  The unit specification must appear in the parameter list, unless
  the unit specifier is the first element in the list.

  The logical unit may already be connected to a file when an OPEN
  statement is executed.  If this file is not the same as the one to
  be opened, the OPEN statement executes as if a CLOSE statement had
  executed just before it.

  If the file to be opened is already connected to the unit or if the
  file specifier (FILE keyword) is not included in the OPEN
  statement, only the blank specifier (BLANK keyword) can have a
  value different from the one currently in effect.  The position of
  the file is unaffected.