What are modules? When do we separate them out? How are they composed?

A list of useful requirements on a meaningful concept of “modules” and their composition is compiled. A system can be composed of different modules, according to different views, and in particular different refinements (decompositions) of the running example of the book, i.e. the organization of a bakery. Here, the modules remain abstract. Later Chapters will show modules that describe specific behavior, items, and data.