In this project, the whole heirarchy is too big to place all in one file, so what is needed is a map that can link together all the seperate files, that's the LinkBase, a central repository for a defined set of files to share a common set of links, both between themselves and to external sites.
You can see that there are some arcs like :
that do not have a xlink:from, this means this link gets added to every page that shares the LinkBase, hence the application-specific xlink:arcrole value.
The rest of the links in here are either locators pointing to external sites like :
xlink:title="The Crudlet.org Open Source Project"
locators pointing to members of the fileset
xlink:title="How the site works, who made it and why, etc."
xlink:href="hrc/about.xml">About the site</loc>
or arcs pointing back to the parent file
The DTD that is loaded into the LinkBase (by the XML Parser) adds default attributes to the tags so the author does not have to.