Opacity is exactly what it sounds like: A measure of the amount of visible light that is blocked by the sooty exhaust of a diesel. In Salt Lake City all cars, including diesels, are required to pass an emissions test. Only one place in the entire city tests diesels, and the only test they run is the opacity test. The standard is 20%, which makes the test entirely meaningless for any modern automotive diesel which is running halfway decent. It's just another example of the government fooling around with stuff they don't understand. I'm sure that in places like California you'd have to pass a "real" emissions test, just like a gasoline engine. The good new is: You'll never flunk the opacity test no matter how old and beat up you TDI gets. At 20% opacity it would be running so bad I doubt you could get it started in the first place.
-mickey