On graphics browsers, the first four icons always appear, though they may be Xed out if there isn't an appropriate document in that direction. I keep the Xed icons around so that the positioning of the icons is consistent from one document to the next. This happens when a document is the root for a number of trees, rather than just one. On text browsers, only those icons which are appropriate will show up. This saves you having to cursor past meaningless links.
Some pages have links going forward, some back, most both.
Those are generally unique to that page, and it should be fairly obvious what they do. If not, try them. Likewise, a couple of pages don't fit cleanly into the tree structure. This whole icon business is an experiment of mine, to get a feel for playing with hypertext and tree-based document structures. I've left them in in case they return, but don't count on them working.