The idea of the example is to demonstrate that it is not needed (mandatory) to pass both objects to the OpenAccessContext. You can create an entire hierarchy (tree) of objects and just pass the root object from the hierarchy to the OpenAccessContext'Add method. OpenAccess will take care for the rest of the objects. Of course, the other approach is also valid, i.e., you can pass both objects (Car and Category) to the Add method.
Generally, there are two types of properties - primitive and navigation. Navigation properties allow you to navigate between two entity types (e.g. Car.Category).
the Telerik team
Q1’11 SP1 of Telerik OpenAccess is available for download
; also available is the Q2'11 Roadmap
for Telerik OpenAccess ORM.