And I'm surprised that position was omitted from the report, because it's probably one of the most common ones for people who sleep on their side. It has to do with maximal muscle relaxation and placing the least amount of tension on the limbs or the back, analogous to the "position of function" that orthopedists try to set your broken hand/wrist/arm in when they put on a cast. (Sublime-To-Ridiculous Dep't: Ever see a picture of Superman flying?!)