Freshly minted from the green sash training manual.
6.9 Fighting Principle # 6 – Line of Attack
One way to approach striking is to think about all the places you can land a punch. You could target the head. You could target the body. You could target the outside of the thigh.
A more analytical way of looking at this is to divide the body into a 3 by 3 spreadsheet. The rows are head, body and legs. The columns are inside, middle and outside. This creates 9 separate squares to strike: inside head, middle head, outside head, inside body, middle body, outside body, inside leg, groin (middle leg) and outside leg.
One could even get more detailed and make a 3 by 3 spreadsheet on just the head, making 9 squares to target on the head. The squares would be… you get the idea. The idea behind line of attack is to get you to think about all the different available targets as well as all the ways you might attack those targets.