External procedures (functions or subroutines) are defined by external subprograms that are not part of any other program unit. External procedures can also be defined by means other than Fortran 90.
External procedures can be invoked by the main program or any procedure of an executable program, and they can be optionally preceded by an OPTIONS statement.
In Fortran 90, external procedures can include internal subprograms (defining internal procedures). An internal subprogram begins with a CONTAINS statement.
An external procedure can reference itself (directly or indirectly).
The interface of an external procedure is implicit unless an interface block is supplied for the procedure.