Natures are not unique. An XML application in a single namespace may have several schemas, even in one schema language. It may have more than one DTD. (XHTML 1.0 has three.) The schema or DTD may be broken up into many modules. Stylesheets, software, documentation, and other related resources may be even more diverse. The specific kinds of resources that are related to a namespace can be further subdivided by purpose. A URI identifying the purpose is included in the rddl:resource element's xlink:arcrole attribute. For example, here are three different rddl:resource elements describing different resources related to MBSML.
<rddl:resource xlink:type="simple" xlink:href="http://developer.megabank.com/xml/spec.html" xlink:role="http://www.w3.org/TR/html4/" xlink:arcrole ="http://www.rddl.org/purposes#normative-reference" > <p> The <a href="http://developer.megabank.com/xml/spec.html">MegaBank Statement Markup Language Specification 1.0</a> </p> </rddl:resource> <rddl:resource xlink:type="simple" xlink:href="http://developer.megabank.com/xml/tutorial.html" xlink:role="http://www.w3.org/TR/html4/" xlink:arcrole ="http://www.rddl.org/purposes#non-normative-reference" > <p> Alan Williams has written a nice <a href="http://developer.megabank.com/xml/tutorial.html"> tutorial</a> describing the MegaBank Statement Markup Language. </p> </rddl:resource> <rddl:resource xlink:type="simple" xlink:href ="http://developer.megabank.com/xml/canonicalization.html" xlink:role="http://www.w3.org/TR/html4/" xlink:arcrole="http://www.rddl.org/purposes#canonicalization" > <p> Before comparing documents, it is necessary to <a href= "http://developer.megabank.com/xml/canonicalization.html"> canonicalize</a> them. </p> </rddl:resource>
All three resources have the same nature (HTML 4.0 document). However, they have different purposes. The first refers to the normative specification of MBSML. The second refers to a non-normative tutorial. The third points to a description of a canonicalization algorithm for MBSML documents.
Purposes tend to be application specific. Table 42-2 lists those purposes that have been identified.
Table 42-2. RDDL Purposes
Natures, purposes, and links together provide enough information for many processes to do something useful with a namespace URL, instead of merely using it as an opaque identifier. Even more importantly, putting HTML at the end of namespace URLs gives us curious humans something plausible to look at when we inevitably type the namespace URLs into web browsers.