The 'simpleContent' element is used to restrict/extend a simple type.

It has either an 'restriction' or 'extension' element as a child.

Use 'extension' when you want to add attributes and have an element value.

Use 'restriction' when you want to add attributes and restrict the element value from a simple type.

Example:

<xs:complexType name="Anchor">
   <xs:simpleContent>
     <xs:extension base="xs:string">
       <xs:attribute name="href" type="xs:anyURI"/>
     </xs:extension>
   </xs:simpleContent>
</xs:complexType>