A Message defines a particular communication between Lifelines of an Interaction, such as raising a Signal, invoking an Operation, or creating or destroying an Instance. Messages specify the kind of communication, the sender, and the receiver.
A Message is represented by arrows between the lifelines. The style of the Message line and arrowhead reflect the types of the Message.
Example of Message in Sequence diagram
Example of Message in Communication diagram
You can see the description of a selected property in the description area of the Specification window. To see descriptions, be sure the Show Description option is selected.