Whenever a question comes up about the Display System in Autodesk Archtitectural Desktop, I always have a hard time with the competing goals of brevity [go on too long, and your audience will drop to zero] and thoroughness [leave out too much and you not only fail to answer the question, you breed confusion and mistrust and generate even more questions].
In the Nov/Dec 2004 issue of AUGI World, Doug Broad has written an article, Wall Styles and Display Reps, that includes one of the best, succinct overviews of the Display System that I have read. If you do not have that issue handy, you can download either a high resolution or low resolution PDF version of the issue by clicking on the links.
You can also get your own subscription to AUGI World, issued bimonthly, by becoming a member of AUGI. Membership is free and gives you access to all of the services AUGI offers the CAD community.