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14.4. 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 natures, 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 almost 20 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 related resources it needs to locate. These are the well-known purposes:
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Validation |
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Schema-validation |
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DTD module |
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Schema module |
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DTD notations module |
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Software module |
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Software package |
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Software project |
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JAR |
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XSLT extension |
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Reference |
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Normative reference |
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Non-normative reference |
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Prior-version |
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Definition |
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Icon |
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Alternate |
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RDDL Directory |
Furthermore, the purpose of an XSLT transform is often the URI for the nature of the resource that is produced by the transform. For instance, the purpose of a stylesheet that converted documents into strict XHTML would probably be http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.
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| 14.3. Natures |  | III. Data-Centric XML |
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