Isn't U what is important?
Underbelly: I hadn't heard of this term, defined by Ron, used explicitly. I was aware of it through secondary supplementary material for large settings however.
The basic idea is that you're playing in a setting (World War II, Star Wars, Babylon 5) where everyone playing knows all the events of the setting 'metaplot' and yet, possibly none of the characters do.
The underbelly is where the characters take part in a plot, that although independent from the major storyline of the setting and focusing on the individual interactions, in some way is related or affects the major meta-plot (most commonly by allowing it to happen - Why if we hadn't done this adventure the millennium falcon might have run out of fuel, etc.) A very useful term for games that take place in already established settings.
Universal: An RPG system designed to be used in multiple settings. i.e. FUDGE or GURPS