Question

Which information do you need to be able to extend a System Sequence Diagram to a Sequence Diagram?

Answer 1

A System sequence diagram visualizes a use case, while a sequence diagram visualizes a method of a class.

The elements participating (exchanging messages) in a system sequence diagram are Actors and Systems. The messages exchanged by these elements could be any type depending on the systems (from web service calls to data input from a human).

The elements participating in a sequence diagram are objects (instances of various classes). The messages exchanged by these elements are method invocations.

First you would analyze and document the use cases. Each use case describes a main business task required. The system use case diagram is part of this activity.

Later you come to design each subsystem, namely each element participating in the system use case diagram. You draw one or more class diagrams depicting the classes participating to the solution. Then you would draw a sequence diagram for the most important (or difficult to understand) methods.