DVD index pages:
Protected means that if you jump to that title rather than following the approved path, you will get kicked out to title 1, chapter 1. Unreachable is a protected track that I never found a route to.
Unreadable is a track that causes an error if you jump directly to it. I believe most of these are tracks that carry program imstructions that are meaningless without context (like the mechanism for running the user-generated slideshow on the Chamber of Secrets extras disc).