Controllers and Subcontrollers - how to set hierarchy?

This may be a stupid question, but... how do you set up a hierarchy of controllers and subcontrollers? I know that the top controller has a blank COMPONENT_ID (according to the code), but where do we specify this when creating a controller? And a subcontroller can have subcontrollers of its own (I think) - what would indicate the top controller in that situation?

Where is all this set? By the time my debugging session kicks in (DO_REQUEST of top controller) the table M_SUBCONTROLLERS has already been set up. I have followed the logic back to the adapter object, and the page object, then I start to get stuck, partly because I don't know what these objects are really for.

It would be nice to know what mysterious entity controls the controller class - calling the methods of that class such as DO_INIT, DO_REQUEST for example.