The entire history of software engineering is that of the rise in levels of abstraction. - Grady Booch

[A class category is] a collection of classes, some of which are visible to other class categories, and others of which are hidden.

Typically, a subclass augments or redefines the existing structure and behavior of its superclasses.

Whereas object-oriented analysis typically focuses upon one specific problem at a time, domain analysis seeks to identify the classes and objects that are common to all applications within a given domain, such as missile avionics systems, compilers, or accounting software.