DEF is not directly related to the amount of soot that goes into the engine. Diesel engines run lean, and because of this, there is excess oxygen in the exhaust gas. During combustion, the oxygen forms with nitrogen to produce Nitrogen Oxides. aka NOx. The job of the ERG is to return some of the exhaust gas back to the combustion event to reduce the amount of excess oxygen available to form NOx. Of course, this reduced mileage and performance on our trucks as well as putting soot back in our engines.
In order to reduce the amount of exhaust gas used in the combustion event, the amount of NOx that is produce during combustion had to be dealt with further down stream. Our trucks now have the Selective Catalytic Reduction system (SCR). It is in the SCR that the DEF gets injected. Through the use of catalytic components and the urea, which is converted to ammonia, the Nitrogen Oxides are converted to just plan ole Nitrogen (N) and water (H2O).