A uniqueness constraint is specified within the element declaration--not the complex type definition.

A uniqueness constraint can be specified for optional content.

It is specified by the 'unique' element--which has a 'name' attribute and contains a 'selector' and one or more 'field' elements.

A declaration:

<xs:unique name="product-name">
<xs:selector xpath="."/>
<xs:field xpath="name"/>
</xs:unique>