product structure can be chosen independently of any sysnet model. A product structure often reflects the structure of the design organization and the design process that is applied. Changes in the product struc- ture are frequently for reasons other than new requirements on concrete systems (which is reflected by sysnet models): a product structure may change due to changes in the development organization, as well as due to design process modifi- cations that are often caused by changes in implementation technology for all or some parts of the system. Since sysnet models are, in principle, unaffected by such changes, it is extremely important that sysnet models and implemented system models be kept well apart. This allows product structures to be modified (which reflects organizational changes and implementation choices) without affecting the sysnet models (which reflect requirements on concrete systems). 1.5.4 How Implemented and Applied System Models Are Related Since the structure of an implemented system depends on the implementation technology, and since the implemented system model is normally described in terms of the products of the product structure, there is not necessarily any one-to- one relation between units that are modelled in the applied system model and prod- ucts of the implemented system. For example, the applied system model may define a unit that in the implemented system model is realized as a small structure of software objects. Other choices made in the implementation system could be to integrate a number of units that are defined in the applied system in a single pro- gram, or to realize a unit partly in software, partly in hardware. Therefore we must carefully separate units that are defined in sysnet models from components that are defined in implemented system models. Components of implemented systems are referred to as "products", which is a generic term only, leaving definition of the exact terminology for components to modelling methods for implemented systems. To produce an imple