EXTERNAL attribute
Standard: (F77) F90 F95 F2003
  Example Program
type, EXTERNAL [ , attr ] :: decl-list

EXTERNAL [ :: ] name [ , name ]...
The EXTERNAL attribute declares that a dummy argument is a procedure, or that an entity is an external procedure or BLOCK DATA subprogram. This attribute is required if the procedure is passed as an actual argument, but not if it is only called.

The syntax above always declares a procedure with an implicit interface; such an interface contains no information about the number and type of arguments. The EXTERNAL attribute is also conferred by an interface block and that declares a procedure with an explicit interface.
Interface statement, INTRINSIC attribute, Type Declaration statement