So I understand your confusion, it would be easy to infer that appendix means a QH cross, but the term's a little more narrow and specific than that.
What you have is a STB X QH cross.
Hard to judge a photo from that angle, but he looks to have a very prominent wither and the typical STB head and that his head and neck tie on high to his shoulder. I'm guessing that he has an incredible disposition with that cross, and is perfect for a hop on and go kind of horse.
Post a photo taken directly from the side if you want a more detailed critique.