The EXTPGM keyword indicates the external name of the program whose prototype is being defined. The name can be a character constant or a character variable. When EXTPGM is specified, then a dynamic call will be done.
If neither EXTPGM or EXTPROC is specified for a prototype, then the compiler assumes that you are defining a prototype for a procedure, and assigns it the external name found in positions 7-21.
Any parameters defined by a prototype or procedure interface with EXTPGM must be passed by reference. In addition, you cannot define a return value.