Thank you for your feedback. I have linked to @dragosroua's post on SBD in this post as he's one of the best sources to learn from about the economics of Steem. The STU is short for Steem Token Unit. He's the developer of Steem.supply app and there's a rewards calculator on its site. The term comes from there. The explanation is given there if you open the link. STU is his own term.