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As you probably know by now, the whole concept behind XML is to enable you to mark up your data semantically. This means that you can have tags, such as <purchase_order>, that describe the data within the tags. This works fine on a local scale (if the application scope is limited), but XML is made for a more global arena: the Web. In the vastness of the Web, <purchase_order> loses its specific meaning, since there are probably innumerable XML documents with <purchase_order> tags. These <purchase_order> tags can then refer to content that may be completely different in terms of logical and physical structure.