Intention here was to create a front-matter/main-matter/end-matter structure for a rendered page, where frontmatter is your heading, navigation bar, logos, etc. This is followed by mainmatter that wraps whatever is slave's content. In the end you have the (surprise) end-matter that holds the footer of the page.
In this case uniqueness needs to be stressed -- there should be only zero or one instance of each portion in a single page, otherwise laying the page out in an arbitrary user agent may become ambiguous (e.g. if you have several front-matters -- which should go first?).
This is intended for the page master -- reusable class descriptors could also be setup to provide "precanned" styles for various other parts of the site, but this is beyond the scope of this TIP. I think it relates more to #5.
Naming of styles in the suggested default-master.adp is, obviously, just that -- a suggestion. If decided they could be anything other than that -- we can call them even __OACS_DEFAULT_MASTER_HEADER_DONT_TOUCH_OR_IT_WILL_BITE__ or whatever else...