Here's the XML:

<library path="lib/libpolygon_plugins"> 
  <class type="polygon_plugins::Triangle" base_class_type="polygon_base::RegularPolygon"> 
    <description>This is a triangle plugin.</description> 
  </class> 
  <class type="polygon_plugins::Square" base_class_type="polygon_base::RegularPolygon"> 
    <description>This is a square plugin.</description> 
  </class> 
</library> 

This is where the library tag provides the relative path of the library that contains the plugins. Similarly, the class tag declares a plugin that will be exported from the plugins library.