My truck (2500HD) has disc brakes AND drum brakes on the rear, with actual brake shoes. I would say that the drum brakes were actual emergency brakes, meaning you could apply the e-brake at speed and safely come to a stop.
My brother's truck (1500) has very thin stamped steel "shoes" with some type of friction material that is about 1/16 of an inch thick. I'd call that a parking brake. If you were to apply his parking brake at speed I'm sure the thin material on the shoes would vaporize in the blink of an eye and disaster would quickly follow.
Mike
