The oversimplification is that type 6 is a softer fiber with more, and more open, dye sites. It can be easily post dyed so is the cheaper option for custom patterns in restaurants and casinos, and as a cheaper fiber it is the first choice for builder grade installations. Easier to dye, easier to remove dye though, and these are the ones most likely to show damage from oxidizer overuse. Hot cleaning solution (with no oxidizer) from a leaky fitting can take the color completely out of type 6, I haven't seen that happen to a 6.6.
Harry,
I may not be able to tell the difference between two new samples, but can almost always tell the difference after they've seen some use. I can't describe exactly how, but there is a combination of feel, loss of tuft twist, pattern definition, dye penetration, construction and how it looks in traffic areas that seem apparent to me. I'd guess this is something like the Oriental rug expert that knows what a rug is, but can't tell you why. You see enough that exactly what you're looking for doesn't register, it just is what it is. I'm sure there are lots of other guys that can do this too, and maybe someone has a way to describe how they do it.