Be sure to visit the Trace Modeler website for updates and UML sequence diagram resources!
UML sequence diagram are used to show the communication between different participants in an interaction.
Here's an example that highlights the interactions in a software system for a simple use case scenario :

You can customize how your diagrams look by changing the various diagram style properties in the 'Diagram style' window.
Trace Modeler supports the following diagram elements :
If you want to find out more, there's a quick introduction to UML sequence diagrams at the Trace Modeler website. Be sure to check out the other content in the resources section as well!