xmlns:xsi XML Schema Namespace Declaration

This is not an attribute, but a namespace pseudo-attribute. The value provides a URI to use for the @xsi:schemaLocation pseudo-attribute. 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 elements: <adoption>, <standard>
Value Meaning
http://www.w3.org/2001/XMLSchema-instance The prefix “xsi” has been set to the URL for W3C XML Schema.
Restriction This attribute has a fixed value which may not be changed in the XML document.
Related Resource
For a discussion on namespaces used in STS, see Namespacing NISO STS Elements.