<schema xmlns="http://www.ascc.net/xml/schematron" >

     <pattern name="Test attribute">

          <rule context="CCC">

               <report test="parent::BBB and not(@id)">Attribute id is missing</report>

               <report test="not(parent::BBB) and @id">Attribute id is used in wrong context</report>

          </rule>

     </pattern>

</schema>