Layers are labeled as subjects, classes and objects, structures, attributes, and services. The best way to determine these was to apply object oriented techniques to realistic examples in the form of pilot studies. At that time, however, not so many methods were available. Our intention was to combine what we considered to be the best of these methods. Modeling the World in Data, Yourdon Press, 1988.
Every object is described by a set of attributes.
At every moment, an object is in some well defined state which can be fully described by the values of its attributes. All possible states of an object are defined in the information model of this object at an abstract level. Sally Shlaer and Steven Mellor. Object-Oriented Systems Analysis . Modeling the World in Data.