An important aspect of XML Security, as well as of the World Wide Web in general, is the ability to point to a wide variety of data objects. As Section 7.1 describes, Uniform Resource Identifiers (URIs) offer a way to achieve this goal. Because URIs can be relative to some assumed base location/name, you need a method for specifying such a base when it might default incorrectly, as noted in Section 7.2. Finally, when you need to point to part of an XML document, you can invoke the expressive power of XPath (discussed in Chapter 6) through the fragment part of a URI reference using the XPointer mechanism, the topic of Section 7.3.
If you are thoroughly familiar with URIs, xml:base, and XPointer, you can skip this chapter. If you are generally familiar with these topics and don't want to get too deep into their details right now, you might consider skipping ahead and referring back to this chapter when necessary.
Note that XLink is a related XML Recommendation [Xlink] that provides a flexible mechanism, including the association of semantics with a pointer. In XML Security, the surrounding XML security structures specify the meaning of a pointer such as a URI. For this reason, XML Security does not need or use XLink.