The EXTERNAL  statement  declares  constants,  variables,  functions,  and
  subroutines  external  to  your  program.  You can describe parameters for
  external functions and subroutines.

  Examples:

  EXTERNAL INTEGER CONSTANT SS$_NORMAL
  EXTERNAL INTEGER Event_flag
  EXTERNAL INTEGER FUNCTION Rect_area (REAL BY REF, REAL BY REF)
  EXTERNAL SUB File_open (LONG BY VALUE, STRING)

1  –  Syntax

  External Constants:

      EXTERNAL data-type CONSTANT const-name,...

  External Variables:

      EXTERNAL data-type unsubs-var-name,...

  External Functions:

      EXTERNAL data-type FUNCTION {func-name [pass-mech] [([external-
               param],...)]}

  External Subroutines:

      EXTERNAL SUB {sub-name [pass-mech] [([external-param],...)]},...

      external-param: [OPTIONAL] [ data-type ] [ DIM ([,]...) ] [ pass-mech]

        pass-mech:  { BY DESC  }
                    { BY REF   }
                    { BY VALUE }