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

@xmlns:xi namespace at the <book> book

<book
  xmlns:mml="http://www.w3.org/1998/Math/MathML"
  xmlns:xi="http://www.w3.org/2001/XInclude"
  xmlns:xlink="http://www.w3.org/1999/xlink"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  book-type="gov"
  dtd-version="2.1"
  indexed="yes"
  xml:lang="en">
 <book-meta>...</book-meta>
 <book-body>...</book-body>
 <book-back>...</book-back>
</book>