xmlns:xlink XLink Namespace Declaration

This is not an attribute, but the namespace pseudo-attribute. The value provides a prefix to use for the XLink linking attributes. All namespace prefixes must be associated with a URL, and the prefix “xlink” has been set to the URL for the World Wide Web Consortium (W3C) XLink Recommendation.
FIXED (not settable) on element: <article>
Value Meaning
http://www.w3.org/1999/xlink The prefix “xlink” has been set to the URL for the W3C XLink Recommendation.
Restriction @xmlns:xlink has a fixed value which may not be changed in the document.
OPTIONAL on many elements; click for list and usage
Value Meaning
http://www.w3.org/1999/xlink When this attribute is used, the prefix “xlink” should be set to the URL for the W3C XLink Recommendation.
Restriction @xmlns:xlink is an optional attribute; there is no default.
Tagged Sample

Allows use of the namespaced XLink attributes

<article dtd-version="1.4d1"  xml:lang="fr"
  article-type="research-article" 
  specific-use="web-services" 
  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"
  xmlns:ali="http://www.niso.org/schemas/ali/1.0/"
>
<front>...</front>
<body>...</body>
<back>...</back>
</article>
Related Resource
For a discussion on namespaces used in JATS, see Namespaces and MathML.