This iteration has defined basic business objects representing significant functional aspects of the system, which represent the data elements of the final system. None of them do any more than provide the basic structure and some business rules around what constitutes a valid structure of those elements, though—There is, as yet, no mechanism for storing those elements, retrieving them or interacting with them, except through their properties, directly in code.
The next iteration chapter will start looking in depth at what is required to provide the storage and state data persistence that the system's applications and service layer will require.