Guideline: Describe a Business Use-Case Scenario in a Business Use-Case Realization
Use a UML sequence diagram to illustrate how business elements interact to realize a use-case scenario.
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Create a UML sequence diagram within the appropriate use-case realization. Identify the element (often a business actor or business worker) that starts the scenario. Place that element on the sequence diagram.

Ask what happens first in the scenario. Who or what does the first element need to communicate with in order to perform the first step of the use case? What needs to get done? There can be multiple operations that need to be performed to realize a single step in the use case.

Place the business element that the initiating element must interact with into the sequence diagram. That element might already have operations associated with it. If so, see if one of those operations will accomplish what needs to be done. If not, add a new operation to the element just placed in the diagram that will accomplish what needs to get done. This is how you identify and document the behavior that the business elements must perform.

Before creating a new operation, check other elements to see if they provide the necessary operation. If so, use them instead of your first choice. It might be possible that the operation you're looking for is actually accomplished by multiple operations across a number of business elements. Place those elements on the sequence diagram and call their operations in the appropriate order. This might be a pattern the business repeats in many areas. Leverage these patterns whenever possible. Business patterns can often be found in the business architecture.

When it's clear that the sequence diagram shows sufficient behavior to perform the first step of the use-case scenario, move to the second step and repeat the procedure. Who's involved next? What needs to happen in order to make the step in the use-case scenario happen?

The scenario has been realized when there are sufficient operations and interactions, in the proper order, for all steps of the use-case scenario to be performed by the business.

See Example: Sequence Diagram Realizing a Use-Case Scenario to see an example of how a sequence diagram realizes a business use-case scenario.

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