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"/>