The kind of relationships is not limited, because any limitation would affect the structure of information. The description of the information is implementation neutral. It uses abstraction to depersonalize objects from the conceptual database design. Objects of the same kind are grouped together to one persistent class. The objects are still at the same granularity level as the global requirements.
Persistent classes contain attributes, which represent the basic element of persistent storage.
An attribute can store any kind of data independent of the later physical limitation. DBMS independent attribute types are used on the logical level to prevent any physical influence. The accessibility of the persistent data is defined by operations on a class. In the design process operations evolve from messages at interaction diagrams. Operations can be public or private, in which case they deal with the internal integrity of the information.