Adding IO flows to a sequence diagram
1. On the Sequence Diagram tab, click the 
IO Flow button.
IO Flow button.2. In line with the required Statement, click the lifeline of the source System Boundary, Instance, Part or Port.
3. Click the lifeline of the target System Boundary, Instance, Part or Port.
4. On the context menu, click the command for the IO Flow you want to add, or click New to create a new IO Flow.
Modeler adds the IO Flow to the diagram.