UML Sequence diagram is mainly designed to tell the interaction between objects in an application in a sequential order, i.e. in the order that these interactions occur.

Normally existing use cases are split into two or more sequence diagrams in the requirement phase. There is an assumption that sequence diagrams are only meant for developers as the class diagrams. But in fact these sequence diagrams are useful for even business users and also for all personnel who ever uses the application. Sequence diagrams give an idea of existing system and how each functionality related to other in the application, and by that the business users can suggest future business changes and can know before hand that which objects may get influenced with the new changes.

The ultimate purpose of this sequence diagram is to explain the sequence of different events in an application which will result some sort of output.

In sequence diagrams, the main importance is given to the order of occurrence of the different actions/events, but not to what is actually happening in every event.

The typical sequence diagram will look like below:

In the above diagram, there are several events (objects, actions, functionalities) that pass messages from one event to other. The diagram exactly gives the order in which the actual application works.

For example, if you take a school:,

- Once the student eligibility process is passed, then only student will get admission.

- Once he gets the admission, then only he pays the fees and go to class.

- After attending the class only, he can write the exams.

- After writing the exams only, he will get marks cards (grade).

In this process, the whole school application works in an order, in a sequence of events that occur one after other. Then only the application works properly. This is where the sequence diagram plays an important role. Sequence diagram aims on these steps. It identifies the order of events that occurs in an application.

You can get more details on sequence diagram, in the below link.

http://www.ibm.com/developerworks/rational/library/3101.html

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Sequence diagram UML

**Sequence diagram (role of Business Analyst)**