A message is attached to an activation when its begin or endpoint is on the activation symbol and not on the object lifeline. Attachment symbols appear at the endpoints of the message arrow (if attachment symbols are not displayed, select , and select the Activation Attachment display preference in the Message category).
You can attach an existing message that touches an activation but which does not display an activation symbol, by dragging the message inside the activation, while holding the Ctrl key down.
When a message is attached to an activation, you cannot move it outside the limits of the activation symbol as shown below:
If you delete an activation with a message attached, the message will be detached from the activation but will not be deleted.
Message Control Flow and Activation
When a message is attached to an activation, the control flow value of the message influences the position of the activation towards the message:
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Procedure Call - A Procedure Call message attached to an activation starts the activation on the receiver lifeline, that is to say the arrival point of the message is located at the top of the activation.
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Return - A Return message attached to an activation finishes the activation on the sender lifeline, that is to say the starting point of the message is located at the bottom of the activation.
Procedure Call and Return messages are the only messages defined on a definite location in the activation: a Procedure Call message is at the top of the activation, a Return message is at the bottom of the activation. Other messages attached to an activation can be moved without any constraint inside the activation.