I will elaborate on the Blogs module and how it is implemented.
It is inhering from the Generic Content, which means that it takes the IContent object functionality. Then we have implemented an IBlog interface and IBlogProvider which can work with Blog objects. Each of this Blog objects has an ID, which we store in the ParentID of the IContent item. That way, we create IContent->IBlog relation.
On the provider level, we can filter the posts (IContent) we take by IBlog, in other words by the same relation.
Does this make sense?
the Telerik team