What is the relationship between NSDocumentController and application delegate?

I am trying to understand the structure of a document based Cocoa application. I understand that the application delegate responds to events from the application (hence its name) and that the document controller manages all documents, opens and saves, etc. What I don't understand is the relationship between the document controller and the application delegate, and possibly the rest of the application.

I am researching this image found here .

The arrow appears to indicate some kind of connection between the application delegate and the document controller.

What kind of connection do they have?

Edit: I understand that this is a very academic question and may not be as necessary, but I want to understand why this is the way it is.