The communication diagram is a simplified representation of the sequence diagram, concentrating on message exchanges between objects within an interaction.

The sequence and communication diagrams are isomorphic. When a communication diagram relates to an interaction already described in a sequence diagram, it is automatically initialized from the information contained in the sequence diagram.

Communication diagram objects are lifelines and messages transmitted by connectors.

When you connect two lifelines with a connector , the connector creation dialog box proposes messages that may be transmitted.

When the connector has been created, you can associate new messages in its properties dialog box, in the Message tab.

The sequence of messages is given by a sequence number associated with each message. See Calculating sequence numbers.

For more details on connectors, see Connectors.