In portal object properties it is possible to specify the name of the default child object used to retrieve the default object, for instance it is used to lookup the default page in a portal. For instance it is possible to specify the default page of a portal using the following :

<portal>
...
   <properties>
      <property>
         <name>portal.defaultObjectName</name>
         <value>default</value>
      </property>
   </properties>
...
</portal>