Shimano components have numbers, presumably to denote quality. E.g. Dura Ace stuff has numbers like 7700, 7800, or 7900. An Ultegra might have 6700 or 6800 on it. A 105 might have the number 5600 or 5700 attached.
Obviously the first number indicates the product-line. What is the second number? Are these the only components with numbers, or do tiagra, sora, and lower get these numbers too?
What does it all mean?