The Bookmark file is just one of the many BeOS filetypes to take advantange of extendable attributes. In case you've been living on a small moon of Jupiter the past few years, I'll explain that web broswers use Bookmarks for quick and easy access back to your favorite web sites (you may also know them as "Favorites" or "Favorite Places").
First, make yourself some bookmarks via NetPositive, if you haven't already. Then, open up your Bookmarks folder. You'll see each bookmark is stored as its own file. However, the entries under the "Name" field probably don't help you know which is which.
That's where extendable attributes come in. Select the Bookmarks sub-menu under the Attributes menu and turn on "URL" and "Title". These now show up in the Tracker window. To change a URL or Title, simple edit the attribute for the proper file. Make folders to better organize your surfing, and drop Bookmarks where it makes sense to you. It's that easy!