I consider it a head of the pec major that cannot be fully isolated but can be stressed. I'm not the only one.
http://www.exrx.net/Muscles/PectoralisClavicular.html
I know that genetic variation determines how isolated your clavicular head of the pec is isolated. I personally can flex my lower chest to almost the exclusion of my upper chest. I can certainly make each head sore to the exclusion of the others.
My favorite exercises for upper chest: Hammer Strength Iso Incline Press, cable flies from low pullies, incline dumbell flies. I also do barbell press, but I never feel it as much. I'm too strong for dumbbell press (j/k). They're too dangerous to handle given my broken lower back.