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There are some important points:

• UC Diagram
• US Cases (Brief,Casual.Fully dresssed)
• SSD ( System Sequence Diagram )
• Contract

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There are some important guidelines

1. New and Improved
2. Essential style
3. Concrete style
4. Write terse use case
5. write black-box use case
6. Take an actor and actor good perspective
7. How to find use case

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The extend relationship provides an answer. The idea is to create an extending or addition use case, and within it, describe where and under what condition it extends the behavior of some base use case.

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Factor out sub function use cases and use the include relationship when:

• They are duplicated in other use cases.
  
• A use case is very complex and long, and separating it into sub units aids comprehension.
  

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• Avoid useless noise to the requirements.
  
• Only focus on what must always be true before the use case starts.
  
• Successful Guarantees focus on what must be true after the use case end.
  

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There are nine main Steps:

1. Scope
2. Level
3. Primary Actor
4. Stakeholder and interest list
5. Precondition and success guarantees (Post condition)
6. Main Success Scenarios and Steps ( Basis flow )
7. Extension or Alternative flow
8. Special Requirements
9. Technology and data variations

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• Brief: One-paragraph summary, usually of the main success scenario.
  
• Casual: Consist of more that one paragraph and each paragraph covers one scenario of the use case.
  
• Fully dressed: This covers detailed requirements of the use case in different section, each section focus on different aspect of the use case such as preconditions and success guarantees.
  

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• Keep the user of the system involved in finalizing its requirements. Use cases are based on the user interaction with system and facilitate it very well.
  
• Use cases are simple. Its story communicate the requirements with less confusion and easy understanding among the stakeholders and development team.
  
• Emphasize on the user goals and perspective by asking them about the system users, scenarios and goals.
  
• Provides layers of sophistication e.g. novice use case, use case diagram, use cases relationships and use case packages etc.
  

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Use Case:

• Text stories to discover and record requirements
  
• Use case name should start with a verb
  
• Use cases are not diagrams, they are text
  
• A use case is a collection of related success and failure scenarios that describe an actor using a system to support a goal
  
• A Set of use case instances, where each instance is a sequence of actions a system performs that yields an observable result of value to a particular actor [RUP]
  

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Actor: Something with behavior e.g. Person, Computer System, cashier etc.

Actors are divided into the following categories:

Primary Actor

• Has user goals fulfilled through using services of the SuD e.g. Cashier
• This type of actor drives a use case

Supporting Actor

• Provides a service to the System under Development e.g. Automated payment authorization service
• These actors clarify the external interfaces and protocols

Offstage Actor

• This actor has interest in the behavior of the use case e.g. Government Tax Agency
• These are identified to ensure that all necessary interests are identified and satisfied.

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There are type of tests for use case:

1. Boss test : For example Login all the day and boss is not happy fails it
   
2. Elementary Business Process Test: Focus on use cases that reflect Elementary Business Process
   
3. Size Test: Is a use case a single step?