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xmlns:xsi XML Schema Namespace Declaration

This is not an attribute, but the namespace pseudo-attribute. The value provides a prefix to use in namespace pseudo-attributes such as @xsi:noNamespaceSchemaLocation. All namespace prefixes must be associated with a URL, and the prefix “xsi” has been set to the URL for the World Wide Web Consortium (W3C) XML Schema.
FIXED (not settable) on element: <article>
Value Meaning
http://www.w3.org/2001/XMLSchema-instance The prefix “xsi” has been set to the URL of the W3C XML Schema.
Restriction @xmlns:xsi has a fixed value which may not be changed in the document.
Tagged Sample

Schema Instance Namespace

The “xsi” prefix is used for attributes and pseudo-attributes defined by the W3C XML Schema Recommendation. 
<article
  xmlns:mml="http://www.w3.org/1998/Math/MathML"
  xmlns:xlink="http://www.w3.org/1999/xlink"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  article-type="research-article" dtd-version="1.3"
  specific-use="migrated"
  xml:lang="en">
 <front>...</front>
 <body>...</body>
 <back>...</back>
</article>
Related Resource
For a discussion on namespaces used in JATS, see Namespaces and MathML.