Note that the words "XML Document" are a hyperlink in one place and bold-face in another. That means that this is a formally defined term in the XML document; the hyperlink points to the definition of the term. In this version of the spec, such definitions are marked with the string "[Definition:]" and (if your browser supports it) color.
When you follow the hyperlink and arrive at the definition of the term, you'll notice that it appears in bold-face; in fact, anything presented in bold-face is a defined term that will have pointers to it from here and there around the document.
These were designed to help out in reading the spec, but have also proved handy in external documents, such as this one. See, we can point at XML Document from here, too!
Copyright © 1998, Tim Bray. All rights reserved.