# Figure 6 2 goal model in the form of an andor tree

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Figure 6-2 Goal model in the form of an AND/OR tree Easy-to-use Well-equipped

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64 6 Model-Based Requirements Documentation Modeling goals with AND/OR trees As the goal model in figure 6-2 shows, the goal “comfortable navigation to destination” is refined into the three sub-goals “dynamic route calculation with respect to traffic congestion”, “comfortable destination input”, and “comfortable route guidance” via AND-decomposition. This depicts that all three sub-goals must be met to consider the super-goal fulfilled. The sub-goal “dynamic route calculation with respect to traffic congestion” in turn is refined by the two sub-goals “manual input of traffic conditions” and “automatic update of traffic data”. The type of decomposition relation depicts that only one of the two sub-goals must be met to consider the super-goal met. 6.3 Use Cases Use cases were first proposed in [Jacobson et al. 1992] as a method to document the functionalities of a planned or existing system on the basis of simple models. The use case approach is based on two concepts that are used in conjunction with one another: Use case diagrams Use case specifications 6.3.1 UML Use Case Diagrams Relations between use cases Use case diagrams in the UML [OMG 2007] (see section 4.2.3 ) are simple models to schematically document the functions of a system from a user’s perspective and to document the interrelations of the functions of a system and the relations between these functions and their environment. Modeling Elements of UML Use Case Diagrams Figure 6-3 shows the most essential modeling elements of use case diagrams, as defined in the Unified Modeling Language (UML) [OMG 2007] .
6.3 Use Cases 65 Figure 6-3 Essential modeling elements of use case diagrams n Use cases: Uses cases that are defined for the system are depicted using oval shapes. These shapes contain the name of the use case. Alternatively, the name can be written beneath the use case. o Actors: Actors are outside the system boundary and represent people or systems that interact with the system modeled. Actors are depicted by a rectangle that receives the name of the actor and is tagged with the stereotype “actor”. If the actor is a person, a stick figure may be used. If the actor is a system, either a rectangle or a stick figure may be used in conjunction with the stereotype “system”. p System boundaries: System boundaries within a use case diagram separate the parts of the use case that are part of the system from the parts (people or systems) that are outside the system boundary. Optionally, the name of the system may be denoted at the system boundary in the diagram. q Extend relation: An extend relation depicts that an interaction sequence that belongs to use case A extends some interaction sequence in use case B at a specified point. This is known as the extension point. The extension is triggered by the condition defined.

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