In UML 2, you can send messages to and from the interaction frame that surrounds your sequence diagram. The frame represents the outer edge of the system (or of the part of the system) being modeled and can be used in place of an actor (actors are no longer used in UML 2 sequence diagrams, but continue to be supported for backwards compatibility in PowerDesigner). A message originating from a point on the frame is said to be sent from an input gate, while a message arriving there is received by an output gate.
In the example below, a high-level sequence diagram, ProcessOrder, shows a series of communications between a user and an sales system:
The message ProcessOrder originates from an input gate on the ProcessOrder interaction frame, and is received as an input message by the Order Controller object. Once the order processing is complete, the message ReturnOrderStatus is received by an output gate on the ProcessOrder interaction frame.
The message CheckOrder originates from the Order Controller object, and is received as an input message by an input gate on the CheckOrder interaction reference frame. Once the order checking is complete, the ReturnCheckStatus message is sent from an output gate on the CheckOrder interaction reference frame and is received by the Order Controller object.
The following diagram shows the CheckOrder sequence diagram which illustrates the detail of the order checking process:
Here, the message CheckOrder originates from an input gate on the CheckOrder interaction frame, and is received as an input message by the Check Interface object. Once the order processing is complete, the message ReturnMessage is received by an output gate on the CheckOrder interaction frame.