the decomp tube is where the def is injected into the exhaust between scr and dpf. looks like there is an ultra sonic density meter in the def tank for quality. they may use other programming and sensors to measure effectiveness of the def being injected like the nox sensor.
First check DEF quality with a refractometer to make sure the concentration is good. Look for NOX sensor codes drifting high or low. Check regeneration frequency in the advanced ecm data page on insite.The spread should be even and around 50 or so hrs. between regens. You can also expand the def code on insite to see nox inlet and outlet numbers outlet should be near zero.