You use an xs:group element to define a model group.
You give it a name that you can then refer to using a QName.
The content is just like xs:complexType in that it must start with xs:all, xs:sequence, or xs:choice.
A declaration:
<xs:group name="ListContents"> <xs:sequence> <xs:element name="item" maxOccurs="unbounded"/> </xs:sequence> </xs:group>
A reference:
<xs:group ref="my:ListContents"/>