The Concept of Fetchabel is that you would dynamilcally access the load and each application is breakup into small modules and all of these accessed thru an single program so as to get reduced load on the resources and also on the plan. I plan a fetchable then it would be associated with rest of all the programs that are in that fetchable.
You can call a fetchable within your PL/I program which is having your Cobol program.
But make sure that the fetchable copybook that you declare on both sides i mean thru PLI on the sender side and in COBOL (Recieving end should match else you would get a S0C7/S0C4.
This process is wisely used when you have PLI as one part of your application and Cobol on the other side of your application.