A defensive gun use is any instance of using a gun to ward off a crime. Just opening your jacket and flashing the butt to a would be robber is considered a DFU. The specific citation used by Professor is just that. A specific type of DFU as opposed to a more generic one.
It's getting increasingly harder to take you gun nutters seriously. Let hope the professor comes back and straightens you gun nutters out. Cause you all are twisting yourselves into a pretzel trying to reconcile the difference between 2 million and 162000 DGU's and it is funny as hell. LMAO.