XML in a Nutshell, 3rd Edition

Purposes

The purpose of a related resource indicates what the resource will be used for. Purposes distinguish between resources with the same natures used for different things. For example, DocBook has multiple XSLT stylesheets for transforming DocBook documents into HTML, XHTML, chunked HTML, and XSL-FO. These are all related resources with the same nature but different purposes. Unlike natures, purposes are optional. You don’t have to use them if you don’t need to distinguish between resources with the same nature, but you can if you’d like.

Purpose names are URLs. These URLs are placed in xlink:arcrole attributes of a rddl:resource element. The RDDL specification defines 21 different well-known purpose URLs, mostly in the form http://www.rddl.org/purposes#purpose. In addition, you are welcome to define your own, but you should use the standard URLs for the standard purposes so that automated software can understand your documents and locate the necessary related resources. These are the well-known purposes:

Validation	http://www.rddl.org/purposes#validation
Schema-validation	http://www.rddl.org/purposes#schema-validation
DTD module	http://www.rddl.org/purposes#module
Schema module	http://www.rddl.org/purposes#schema-module
DTD notations module	http://www.rddl.org/purposes#notations
DTD entities module	http://www.rddl.org/purposes#entities
Software module	http://www.rddl.org/purposes#software-module
Software package	http://www.rddl.org/purposes#software-package
Software project ...