What is a PROC and why do people say not to copy the whole thing?

My job has // EXEC MYCOMP. Someone said MYCOMP is a PROC. Where does the actual JCL live and how do I override one DD?

PROC = cataloged JCL recipe. Lives in a procedure library. Your job EXEC's it and only overrides what you must.

Override example style: //STEP1.SYSIN DD DSN=MY.SOURCE(MEMBER),DISP=SHR

Do not copy the whole PROC into your job unless you have a reason. You inherit fixes when the PROC updates.