Categorising Qi has cropped in in many contexts and there appear to be many 'models' - no doubt all fit for a specific purpose.
There's a very good book (as above) which runs to almost 200 pages and describes the various meanings of just that one word! http://www.amazon.com/gp/product/0912111631/sr=8-1/qid=1147780446/ref=sr_1_1/103-9246775-9680667?%5Fencoding=UTF8
A reviewer who gives it (rightly so I feel) 5* says (snip):
"After reading this work, one realizes that one's understanding of qi can never be complete, and will always be dictated by the context. Appropriately, this book both expanded my awareness of the concept of qi, enhanced my understanding, yet failed to provide a coherent, simplistic answer.
Before reading this book, I had come to the conclusion that there really was no such thing as qi. I was right, and I was wrong. But I feel significantly closer to true
This is a must read for anyone seeking to understand the concept of Qi, that has yet to find the answer, or is not sure of the answer. Or anyone who is interested in better understanding the root of Chinese culture and thought."
So remember that next time you think you have this "Qi-thing" sussed lol.
Anyway a snip from that book:
"The Differentiation of Qi in Anatomy and Physiology
The Yellow Emperor's Classic of Internal Medicine
records over 270 different classifications of qi. These divide into two categories... yin qi and yang qi...
The most yin of the yin qi is known as ying qi... This is the nutritive substance that is derived from the food we eat by the digestive system and circulated throughout the body by the blood. The most yang of the yang qi is know as wei qi or defensive qi. Wei qi circulates outside of the jing luo and serves as the outermost defensive mechanism of the body