Xanadu allows transclusion, which is the existence of the same information in more than one place. Two completely different documents can share a few paragraphs, for instance, and when those paragraphs are updated on one of the documents, the new version is automatically seen on the other. Ted Nelson sees transclusion as "what quotation, copying and cross-referencing merely attempt."
In Xanadu structure, a link is a connection between things which are different, and a transclusion is a connection between things which are the same. This is explained in The Future of Information by T. Nelson, and in various articles to be found in our archive at Xanadu Australia.