They are declared using the xs:attributeGroup element.
You give them a name that you can then refer in element declarations or attribute group definitions.
They can contain some number of xs:attribute or xs:attributeGroup elements.
A declaration:
<xs:attributeGroup name="linkingAttributes"> <xs:attribute name="href" type="xs:anyURI"/> <xs:attribute name="content-type" type="xs:string"/> </xs:attributeGroup>
A reference
<xs:attributeGroup ref="my:linkingAttributes"/>