A Message in a Communication diagram is equivalent in meaning to a Message in a Sequence diagram. It implies that one object uses the services of another object, or sends a message to that object. Communication Messages in Enterprise Architect are always associated with an Association link between object instances. Always create the Association link first, then add Messages to the link.
Messages can be dragged into a suitable position by clicking and dragging on the message text.
Communication Messages should be ordered to reflect the sequencing of the diagram. The numbering scheme should reflect the nesting of each event. A sample sequencing scheme could be:
2, 2.1, 2.2
This would indicate events 2.1 and 2.2 occur within an operation initiated by event 2.
If the target object is a class or has its instance classifier set, the drop-down list of possible message names includes the exposed operations for the base type.