This is a neat question, I hope it gets responses from some knowledgeable people because I'd like to learn too.
Here is a post that I found helpful on the subject (with diagrams and pictures and everything!): What is a “sound” dog? | Ruffly Speaking
Pat Hastings has a book called "The Puppy Puzzle" that talks about these things too. I'll admit that I still don't fully understand it (and I've read the thing TWICE) but I'm a very poor visual learner so it takes me a while.
ARCHX TDCH Pongu the Insane, CD-C, RL1X3, RL2X2, RL3X (GSD mix, b. Apr 2010)
Crookytail the Tigerwuff, RL1, NTD (Akita mix, b. Jan 2011)