uction to Systems Modelling figuration aspects for software and hardware, including mechanical structures and cabling. Figure 1.10 gives some examples. A sysnet standardized describes model system A collaboration diagram refines A sysnet describes applied model system realizes software describes A subsystem describes, system structure implemented model system hardware isAnInstanceOf system describes An installation model describes concrete system A cell layout plan Fig. 1.10 Different system properties described on different levels of system abstractions Although SM&AML only supports the creation of sysnet models, it is important for a development process that uses SM&AML (and for model management) that the relations between models of different system abstractions are well understood and carefully managed. In this section, we will therefore discuss model manage- ment in the context of SM&AML. Since the difference between information struc- tures and system structures is sometimes not well understood in methods for systems engineering, we will make an important distinction between the following types of relations: The relations refines, realizes, isAnInstanceOff between system abstractions, The communication relations between interacting elements in a system, Information relations (IE relation), which are relations between information ele- ments in an information structure. 1.5.1 The Structure of System Models From system management point of view, a system model is logically a single information element (IE). Considering the system abstractions we have defined, the structure of system models will be as shown in Fig. 1.11. The structure consists of information elements and relations between information elements. The IE rela- tions should not be confused with relations between system abstractions (refines, realizes and isAnInstanceOf). System relations can be quite complex, and must be described as separate information elements, in order to support requirement and 1.5 System Models fault tracing. It is important tha