The outer assembly is the one on the quarter panel also we call it tail lamp, while The inner assembly is the one on the trunk with another name back lamp.
And here in ITN Auto one set taillights includes two outer and two inner taillights, also two tail lamps and two back lamps. These are usually for sedans and hatchbacks with a third door or a fifth door.
ANd for some vehicles especially he SUV and Pickups, the taillamp is taillight on the body not on the trunk, in our listing here one set is one pair, two pieces, left and right.