welcome to the wonderful world of slipjoints....
the stockman is so called because it was and is carried by stockmen/cattlemen/cowboys. With a general purpose clip blade, a sheepsfoot for cutting rope and leather straps and a spey blade for castrating livestock this is the knife many ranchers keep close at hand. On some models the spey blade is even marked FLESH ONLY. A variant on the Stockman knife is the Cattle knife which has a similar blade configuration but an even rounded handel shape, like a Swiss Army Knife.
The Trapper is so called because it was and is carried by hunters and trappers. With a general purpose clip blade and a spey blade for skinning this knife can clean a buck or dress a rabbit or whatever game you catch.
Just my limited knowledge which I picked up here, I am sure some of the more knowledgable fellas can add more.
BTW the above is one of the many reasons I LOVE slipjoints - each model has a legacy to it, call me a romantic fool but when I pick up a stockman I feel a connection to the cowboy spirit!