mented system from an applied system model means to take design decisions on how to realize the units and communication relations that are defined in the applied system model. realizes relation documents all those deci- sions. It is important that such a document is regarded as a part of the implemented system model and not of the applied system model, since any number of imple- mented systems can be created out of a single applied system (by different realiza- tion choices or by different views on the product structure). However, some questions still remain: how do we describe the realizes relations, and why should these relations be documented at all? Figure 1.14 illustrates the first question. 1 Introduction to Systems Modelling Implemented system model System model structure Applied system model model applied system unit names refines refDes realizes product system productStructure implemented realDes system product Fig. 1.14 Products, units and the realizes relation Assuming that the implemented system model is managed as a hierarchical structure of model fragments (i.e. a product structure), the realizes relation between an implemented system and an applied system also becomes fragmented. Each product information element will have a realizes relation attribute (realDes) that describes which part of the applied system model the product realizes, and how. The set of all these information elements, integrated over the whole product struc- ture, now constitutes the complete description of how the applied system model is mapped on the particular implemented system model. Note that SM&AML does not deal with how to describe realizes relations. It is in the nature of the problem that this depends on modelling methods and languages that are used by design processes for creating implemented systems models. The answer to the second question is: in managing a complex, long-lived sys- tem, we always need requirement tracing. This need works both ways: require- ments on new services, functions and improved characteristic