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xmlns:xi XInclude Namespace Declaration

This is not an attribute, but the namespace pseudo-attribute. The value provides a namespace prefix and URI to use for the XInclude elements. All namespace prefixes must be associated with a URL, and the prefix “xi” has been set to the URL for the World Wide Web Consortium (W3C) XInclude Recommendation.
FIXED (not settable) on many elements; click for list and usage

<adoption>, <app>, <app-group>, <standard>, <xi:fallback>, <xi:include>

Value Meaning
http://www.w3.org/2001/XInclude The prefix “xi” has been set to the URL for the W3C XInclude Recommendation.
Restriction This attribute has a fixed value which may not be changed in the XML document.