A Communication diagram is a type of Interaction diagram, which falls under the behavioral diagramming family. The Communication diagram illustrates the various static connections between objects, and models their interactions. It also presents a collaboration that contains a set of instances as well as their required relationships given in a particular context, and includes an interaction that defines a set of messages. These messages specify the interaction between the classifier roles within a collaboration that will serve to achieve the desired result.
A Communication diagram is given in two different forms: at the instance level or at the specification level.