You can assign a Signal only to Send or Call (synchronous or asynchronous) Messages. Only one Signal can be assigned to a Message.
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If a Signal is assigned to a Call Message, the Message is automatically converted to a Send Message.
To assign a Signal to a Message by dragging
Select a Signal in the Containment tree.
Drag the selected Signal to a desired Message.
To assign a Signal to a Message by typing Message name
Click the Message to type the Message name on the diagram. A Signal is automatically created with the same name as Message name.
To assign a Signal to a Message from the Message Specification window
Draw a Send Message between two Lifelines or select an existing one on a diagram pane.
Open the Message Specification window.
Click the Signature (signal) property value cell and then select a Signal in the drop-down list.
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Names of signal attributes are displayed in the message signature by default. However, you can change this behavior by opening the Symbol Properties dialog (right-click a Message and select Symbol Properties) and setting the Show Attribute/Parameter Names property to true to display them or false to hide them.