Identifying information at an atomic level
The W3C's Namespaces in XML Recommendation provides XML users with a way to uniquely and controllably identify the vocabularies they are using inside of documents.
Namespace identifiers forged from URIs
Namespace identifiers use the URI syntax defined in the IETF's RFC 2396. In some regards, URIs are just URLs and URNs, but their use as identifiers brings both flexibility and complexity.
Because URIs can be rather verbose, and because they can contain characters which aren't permitted in XML element names, namespaces use prefixes which map to URIs as a convenient shorthand. (A default namespace can be used for names without prefixes.)
Because documents may contain multiple namespaces, and because the possibility of collisions between prefixes exists, namespaces allow developers to map prefixes to URIs for elements and their contents, not just document-wide.
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