<self-uri> is used when a document or document component (such as an article or a chapter) is available in multiple forms, for example, as a means of indicating the online version of a print document. Conversely, a URI that indicates a different book, article, or any other kind of live-linked external reference is usually tagged as <ext-link>. The <uri> element, although allowed everywhere <ext-link> is used, is more typically part of an address, information concerning a contributor, or part of a reference citation. Loosely put, an external link <ext-link> element is intended to act as a link; a URI <uri> element identifies a URI (such as a URL) in the text, but may or may not be a traversable link; and the <self-uri> points to another form of the same document. To make any of these elements into a live link, add the XLink @xlink:href attribute.