<topic> elements can indicate a single concept:

 <topic access=point="identity-number" value="1969.11.1"></topic> 

or they can contain a complete sub-structure:

<topic access-point="object.work" value="Storm-tossed Frigate">
 <context CHIO="creation"><context CHIO="creator">
  <topic access-point="person" value="Chambers, Thomas" 
  ROLE="artist"></context>
  <topic access-point="date-range" FROM="1825" TO="1874" 
   EXACT="NONE"></topic>
 </context>
 <context CHIO="current-location">
  <topic access=point="identity-number" value="1969.11.1"></topic>
 </context>
</topic>