In practice requirements are often documented using

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In practice, requirements are often documented using natural language. However, it can be observed that requirements are increasingly often documented using models. Requirement models are used in addition to natural language requirements documentation and partly replace require- ments that would have been documented using natural language. 6.1 The Term Model Models as abstracting images from reality A model is an image that abstracts from reality or that serves as a abstracted representation of reality that is to be created. Modeling may be applied to material or immaterial objects of an existing reality or a reality to be devel- oped. Similarly to [Stachowiak 1973] , we define the term model as follows: Definition 6-1: Model A model is an abstract representation of an existing reality or a reality to be created.
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60 6 Model-Based Requirements Documentation 6.1.1 Properties of Models Every model possesses three important properties that are also the preva- lent advantages of models: Mapping of reality : Every model maps certain aspects of the observed reality onto its modeling elements. Model creation can be descriptive and prescriptive in nature. In the case of descriptive model construc- tion, the resulting model documents the existing reality. In the case of prescriptive model construction, the resulting model serves as a proto- type for a fictitious reality. Depending on the perspective, models themselves can be both descriptive and prescriptive at the same time. For example, a model is descriptive with regard to the conception of the stakeholder who is constructing it and prescriptive with regard to the system to be developed. Reduction of reality : Models reduce the mapped reality. It is common to differentiate between selection and compression. During selection, only particular aspects that are part of the universe of discourse of the system are modeled. In contrast, aspects of the subject-matter of the system are summarized during compression. Pragmatic property : A model is always constructed for a special pur- pose and within a special context. The purpose of the model may affect the construction and the purpose-driven reduction of reality within the models. Ideally, a model contains only the information necessary for the respective purpose. Typically, graphical models are used in requirements engineering. Their modeling elements are conceptualizations of material or immaterial objects, or people, in reality. 6.1.2 Modeling Languages Syntax and semantics In order to construct conceptual models, specific modeling languages are used. A modeling language is defined by its syntax and semantics: Syntax: The syntax of a modeling language defines the modeling ele- ments to be used and specifies the valid combinations thereof.
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