The element 'complexContent' can be use to specify derivation.

It can also be use to wrap simple definitions that aren't derivations.

These are the same:

<xs:complexType name="Pair">
   <xs:sequence>
      <xs:element name="A" type="xs:string"/>
      <xs:element name="B" type="xs:string"/>
   </xs:sequence>
</xs:complexType>
<xs:complexType name="Pair">
   <xs:complexContent>
      <xs:restriction base="xs:anyType">
        <xs:sequence>
           <xs:element name="A" type="xs:string"/>
           <xs:element name="B" type="xs:string"/>
        </xs:sequence>
      </xs:restriction>
   </xs:complexContent>
</xs:complexType>

Usability?