A Sequence or Collaboration diagram is modeled for every scenario in the system. Whereas the Use Case diagram models a business view of the scenario, the Sequence diagram contains implementation details of the scenario, including the objects and classes used to implement the scenario and the messages passed between the objects.

Typically one examines the description of the Use Case to determine what objects are necessary to implement the scenario. If you have modeled the description of the Use Case as a sequence of steps, then you can 'walk through' the steps to discover what objects are necessary for the steps to occur.

The Sequence diagram uses vertical lines to represent objects and horizontal vectors to represent messages passed between the objects. At first, you might want to label the message lines in the Sequence diagram with the business name of the message, similar to the Use Case step. As your design progresses, you might want to label the message lines with the name of a method that one object is asking the other to invoke.

The Collaboration diagram represents an alternate view of the Sequence diagram. The Collaboration diagram shows how the objects in the scenario interrelate and gives the modeler the ability to show detail, such as visibility. Collaboration diagrams are good for understanding all of the effects on a given object and are good for procedural design.

To show the implementation details of a scenario, you can choose to draw either a Sequence or a Collaboration diagram, or both. System Architect automatically creates a Collaboration diagram when you create a Sequence diagram, and vice versa. The two diagrams are always kept in synchronization, so that you can do all of your drawing in one diagram, and then change views to see the changes in the other.