If I understand correctly, you're asking about pork butt and pork shoulder. If that's indeed the case, they're actually the same thing. On a whole pork shoulder, they're often cut in half at the processor to break them down into two smaller components. The bottom half is referred to as a picnic ham, and the top is referred to as a pork butt, Boston butt, and I'm sure there are likely other names for it as well. Most people cook the top half, or the butt. It has great marbling and is outstanding for long, slow cooks due to the fat content.
Not sure if this helps?