Messages indicate a flow of information or transition of control between elements. Messages can be used in Timing diagrams, Sequence diagrams and Communication diagrams (but not Interaction Overview diagrams) to reflect system behavior. If between Classes or classifier instances, the associated list of operations is available to specify the event.
Moving a Message can disrupt the organization of other features on the diagram. To avoid this, and move only the Message, presswhile you move the Message.
OMG UML Specification:
The OMG Unified Modeling Language specification, (v2.5.1, p.623) states:
A Message defines a particular communication between Lifelines of an Interaction.
The OMG Unified Modeling Language specification, (v2.5.1, p.574) also states:
The signature of a Message refers to either an Operation or a Signal. The name of the Message must be the same as the name of the referenced Operation or Signal.