That a law directive that was considered to be

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that a law directive that was considered to be relevant for the system to be developed no longer impacts the system or is no longer considered rele- vant. The system context is therefore reduced ( 1 in figure 2-4 ). If a new law directive is identified that influences the system, the system context is extended accordingly ( 2 in figure 2-4 ). Figure 2-4 Gray zone between system context and irrelevant environment Gray zone between system context and irrelevant environment Since the context boundary separates the system context from those parts of reality that are irrelevant to the system, a complete and precise definition of the context boundary for complex systems is virtually impossible. In addition, it may not be possible to clarify for single aspects of the environ- ment whether they influence the system to be developed or are influenced Definition 2-3: Context Boundary The context boundary separates the relevant part of the environment of a system to be developed from the irrelevant part, i.e., the part that does not influence the system to be developed and, thus, does not have to be considered during requirements engineering.
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2.3 Documenting the System Context 17 by it or not. These two observations are the reason for the existence of a gray zone with regard to the context boundary (see figure 2-4 ). Resolving and shifting of the gray zone This gray zone therefore comprises identified aspects of the environ- ment for which it is unclear whether they have a relation to the system or not. In contrast to the gray zone between the system and the system con- text that must be resolved in the course of requirements engineering, it is not necessary to resolve the gray zone between the system context and the irrelevant environment entirely. 2.3 Documenting the System Context In order to document the system context (especially the system and con- text boundaries), “use case” diagrams [Jacobson et al. 1992] (see sections 4.2.3 and 6.3.1 ) or “data flow” diagrams [DeMarco 1978] (see section 6.6.1 ) are often used. When the context is modeled with data flow dia- grams, sources and sinks in the environment of the system that represent the source or destination of data flows (or flows of material, energy, money, etc.) are modeled. In use case diagrams, actors (such as people or other systems) in the system environment and their usage relationships to the system are modeled. To model the system context, UML class diagrams [OMG 2007] (see section 6.5.2 ) may also be used. In order to document the system context of a system as thoroughly as possible, typically several documentation forms are used. 2.4 Summary The system context is the part of the reality that influences the system to be developed and thus also influences the requirements for the system. In order to be able to elicit the requirements for the system to be developed, it is necessary to define the boundary of the system to the system context and the boundary of the system context to the irrelevant environment first.
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