All of the above, with shoulders, with back, by themselves. It varies.
Traps are a back muscle, true, but that 15% ontop is heavily involved with the scapular elevation movements, which shoulder days excite more of. It's also the most prominent portion of the traps that are visible with "shirt on" or beater. The scapular retraction movements, which would be the remaining 75%, will excite your posterior traps to swell as if they're being worked, but not in that scapular elevation kind of way.
I personally prefer hitting my traps with a back day for the very reason that traps are involved in every other back exercise I perform, but they seem to get a deeper stimulation when I do traps on a shoulder day. Much more elevation going on and direction stimulation of the posterior traps.