Generalization and specialization are fundamental concepts in domain modeling that support an economy of expression; further, conceptual class hierarchies are often the basis of inspiration for software class hierarchies that exploit inheritance and reduce duplication of code. Association classes capture information about an association itself. Time intervals capture the important concept that some business objects are valid for a limited time. And packages are a way to organize large domain models into smaller units. Most of these concepts are introduced in the context of the NextGen case study; a refined Monopoly domain model is shown starting on p. 538.
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