Your initial understanding is quite okay I would say:
000 Empty - Base Publication, Empty Parent, we use this Publication to make sure all future created Publicationd, can have at least a single common parent, so everything belongs to a single BluePrint. Because we keep this Publication "empty" (i.e. no items other than the defaults), it will never harm to use it as a parten for any new Publication.
100 Master - Schema, Category/Keywords, Template Building Blocks, HTML Design and Settings for modules. Like the Schema Publication you see in most BluePrints, but it also contains Templates, since with DXA there is no need to separate design out in a different Publication (as all our Templates are the same anyways, it is only the views and HTML designs that would differ).
110 DXA Site Type - XPM Page Types (incl. prototype Components), XPM Content Types, Include Pages (Header, Footer, etc.), Home Page, Error Page, Structure Groups (system\assets subtree) and our default Business Process Type. This Publication is marked as a Site Type, for use in the Site Wizard in Web 8, so you can create "empty" DXA Publications based on it.
200 Example Content - Example content, what the name says basically
400 Example Site - Example website, what the name says basically
For your further questions, why is 200 Example Content inheriting from 110 DXA Site types?
Well because 110 DXA Site types contains all of the items required for a default DXA Publication. For the Content, thatbasically is the Schemas and Folders (Schemas inherited through 100 Master of course). But it also shows the BluePrint you would create when you want to make a new microsite in DXA. You would create a Content Publication and a Site Publication. Both would inherit from 110 DXA Site types, and the Site Publication, would also inherit from the Content Publication. That way you can create translated Content Publications, and thus translated Sites.
