Adding reply messages to a sequence diagram
1. On the Sequence Diagram tab, click the 
Reply button.
Reply button.2. In line with the required Statement, click the lifeline of the source object, that is, the Package or instance whose type owns (or is going to own) the Operation, or a Part or Port that owns (or is going to own) the Operation
3. Click the lifeline of the target object, that is, the System Boundary, a Package, Instance, Actor, Part or Port.
4. From the Selection dialog, select the required Operation, and then click OK; or click the New button.
Modeler adds a reply message to the diagram.
5. If you want to set instance properties of the reply message:
a. Right-click the reply message, and then click Instance Properties.
b. On the Actual Parameters tab, specify any Actual Parameters.
c. On the Options tab, specify the Return Value, Variable Name and Latency as required.