Soot trap is just what diesel guys call it. Its nearly the same thing as a Catalytic converter. Or Catalyist converter or Cat I can't spell and my chemistry is so rusty I barely remember H2o is water. Anyway its a honycomb looking material that gets hot in the exhaust and reacts with the exhaust gases to react and remove some oxide or other. Something about the Nitrogen in air getting so hot in combustion and reacting and creating Nox's etc. Not sure how different it really is from a gasser CAT similar but probably slightly different. You might find a technical post on it. But CAT is used commonly in a lot of post and is too short so not sure of good search word phrase.
It surely doesn't keep a diesel from belching black smoke or trap soot from escaping by intent. More or less it does restrict the exhaust gasses and is kinda like a trap maybe why its called a soot trap because it seems to soot up and foul over time with a very poorly running engine. Or maybe that it traps the exhaust gas to increase heat for the catalyst reaction?
97 5spd K2500 Ext Cab short Bed ~160K miles.
TM, 3" downpipe & 4" exhaust, remote FSD, remote oilfilter, Gauges: EGT, Boost, Fuel Pressure, B&W Gooseneck Turnover ball, Prodigy Brake Controller. Hi-Temp Hydraulic Oil Cooler Lines.