Attributes can be declared in the 'complexType' element via the 'attribute' element.
Example:
<xs:attribute name="href" type="xs:anyURI"/>
The 'use' attribute specifies optionality and can have values:
optional - the attribute doesn't have to be present.
required - the attribute must be present.
prohibited - the attribute is prohibited from being used (for type derivation cases).
Don't worry about the value 'prohibited' for now...
In context:
<xs:element name="link"> <xs:complexType> <xs:attribute name="href" type="xs:anyURI"/> </xs:complexType> </xs:element>