From the VS2010 MSDN sequence diagram documentation…
Relationship to other diagrams
You can use UML sequence diagrams together with other diagrams in several ways.
Lifelines and types
The lifelines you draw in a sequence diagram can represent typical instances of the components or classes in your system. You can create lifelines from types, and types from lifelines, and show the types on UML class diagrams and UML component diagrams. For more information, see Classes and Lifelines.
Beside we can create lifelines from/for classes we also can create lifelines for/from components… which is actually a very interesting feature.
Context menu on a component shows the create lifeline option [not available when clicking on a part ]
After the creation of several lifelines, you can draw the interactions between the components. There isn’t functionality like the bi-directional method creation in the class-sequence diagram interaction for the component-sequence diagram. Kind of logic, a component only has interfaces.
Why would you want to do this, create sequence diagrams for components?
While the component diagram is a static view of your system the sequence diagram is dynamic, it visualizes the flow in your application, the sequence of actions that occur in the system. So, you could use sequence diagrams to validate your system, your component design. Take a scenario, a requirement scenario, and create a sequence of actions from in a sequence diagram . By doing this you create a kind of simulated behavior of the component structure, which you can check on correctness and consistency.
The scenarios used for this can be found in the use case diagrams , which describe the actions that take place on/in the system. Now describes a use case diagram not really a scenario, you better can use an activity diagram for that , which can be based on the use case diagram as a kind of use case realization. Now the interesting thing is, testers use activity diagrams to create test cases, they have specific technologies to extract the right amount and type of scenarios [see http://robkuijt.nl/index.php?entry=entry080423-135750 ], these scenarios we can use to validate our component design which actually closes the loop.
It would be even more interesting and powerful if we not only could link sequence and component diagrams but also use case and activity diagrams making notification when an activity changes and a scenario can’t be run anymore on the purposed component… hard to accomplish, but interesting, versioning would be hard…