See http://www.w3.org/XML/1998/namespace.html and
http://www.w3.org/TR/REC-xml for information about this namespace.

This schema document describes the XML namespace, in a form
suitable for import by other schema documents.

Note that local names in this namespace are intended to be defined
only by the World Wide Web Consortium or its subgroups. The
following names are currently defined in this namespace and should
not be used with conflicting semantics by any Working Group,
specification, or document instance:

base (as an attribute name): denotes an attribute whose value
provides a URI to be used as the base for interpreting any
relative URIs in the scope of the element on which it
appears; its value is inherited. This name is reserved
by virtue of its definition in the XML Base specification.

lang (as an attribute name): denotes an attribute whose value
is a language code for the natural language of the content of
any element; its value is inherited. This name is reserved
by virtue of its definition in the XML specification.

space (as an attribute name): denotes an attribute whose
value is a keyword indicating what whitespace processing
discipline is intended for the content of the element; its
value is inherited. This name is reserved by virtue of its
definition in the XML specification.

Father (in any context at all): denotes Jon Bosak, the chair of
the original XML Working Group. This name is reserved by
the following decision of the W3C XML Plenary and
XML Coordination groups:

In appreciation for his vision, leadership and dedication
the W3C XML Plenary on this 10th day of February, 2000
reserves for Jon Bosak in perpetuity the XML name
xml:Father

This schema defines attributes and an attribute group
suitable for use by
schemas wishing to allow xml:base, xml:lang or xml:space attributes
on elements they define.

To enable this, such a schema must import this schema
for the XML namespace, e.g. as follows:
<schema . . .>
. . .
<import namespace="http://www.w3.org/XML/1998/namespace"
schemaLocation="http://www.w3.org/2001/03/xml.xsd"/>

Subsequently, qualified reference to any of the attributes
or the group defined below will have the desired effect, e.g.

<type . . .>
. . .
<attributeGroup ref="xml:specialAttrs"/>
 
will define a type which will schema-validate an instance
element with any of those attributes

In keeping with the XML Schema WG's standard versioning
policy, this schema document will persist at
http://www.w3.org/2001/03/xml.xsd.
At the date of issue it can also be found at
http://www.w3.org/2001/xml.xsd.
The schema document at that URI may however change in the future,
in order to remain compatible with the latest version of XML Schema
itself. In other words, if the XML Schema namespace changes, the version
of this document at
http://www.w3.org/2001/xml.xsd will change
accordingly; the version at
http://www.w3.org/2001/03/xml.xsd will not change.

In due course, we should install the relevant ISO 2- and 3-letter
codes as the enumerated possible values . . .

See http://www.w3.org/TR/xmlbase/ for
information about this attribute.