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extension
subject			fact

extension	has definition A use case that makes optional interactions explicit or handles exceptional cases
	has purpose to make optional interactions explicit or to handle exceptional cases, therefore allowing the description of the basic use case to remain simple
	is a subtopic of 7.3 - Developing Use Case Models of Systems
	is related to the use case it extends
	is a kind of use case
	lists all the steps from the beginning of the use case to the end, including the handling of the unusual situation
use case	assists an architect with the first draft of the architectural model
	can be used to plan the development process
	can be used to structure user manuals
	can form the basis for the definition of test cases
	can serve as as part of the contract between the customers and the developer
	describes the user's interaction with the system
	does not describe the computations the system performs
	give an architect an idea about which components will be needed and how they will interact
	has a use case name
	has actors the actors or actor who can perform this use case (optional)
	has goals zero or more goals
	has postconditions zero or more postconditions
	has preconditions zero or more preconditions which describe the state of the system before the use case occurs
	has related use cases zero or more use cases that may be generalizations, specializations, extensions or inclusions of this one
	has steps each step of the use case using a two column format, with the left column showing the actions taken by the actor, and the right column showing the system responses
	has summary
	includes only actions in which the actor interacts with the computer
	is used to develop requirements
	is used to validate requirements
	may have high political or commercial value
	may have one or more preconditions
	may represent a high risk because for some reason their implementation is problematic.
	must be validated using requirements validation methods
	normally includes only actions in which the actor interacts with the system
	should be independent of any particular user interface design

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