Map the control flow. This code/app/whatever is doing something in production right now. What tells it to start? How does the control flow from the start point to the stuff that takes in data to the stuff that writes the output or does whatever this app does? Whatever the options for how it works are, where are they set, how do they make it into the core of the application to affect whatever it does?
Map the data flow. Input must be coming into this thing somewhere. Find where it reads it in, where it writes it out, and how it gets from one to the other, what data structures and methods it passes through on the way.
Map the control flow. This code/app/whatever is doing something in production right now. What tells it to start? How does the control flow from the start point to the stuff that takes in data to the stuff that writes the output or does whatever this app does? Whatever the options for how it works are, where are they set, how do they make it into the core of the application to affect whatever it does?
Map the data flow. Input must be coming into this thing somewhere. Find where it reads it in, where it writes it out, and how it gets from one to the other, what data structures and methods it passes through on the way.