if you get the online book "Who knew you could do that with RPG/IV: A scorcer's guide to RPG" you will find some information on this.
bob also has a printed book "RPG TnT: 101 Dynamite Tips 'n Techniques with RPG IV" that has some examples you can find examples of using sub procedures and prototyping.
for a very in-depth (and reusable) resource, i reccomend the DVD set that bob has created that explains procedures, sub procedures and service programs/modules and prototyping.
In my opinion, under a module project:
The prototype of a procedure is better using extname to define.
There are three kind of copybooks for procedures. First copybook is Pr definition for a procedure to match Pi definition without EXTPGM or EXTPROC keyword; Secode copybook is Pr definition for EXTPGM; Third copybook is Pr definition for EXTPROCE.