Communication Diagrams show communications in a system, emphasizing the structure and the communication paths.

A communication diagram focuses on the organization of the objects that participate in an interaction. They are the primary source of information used to determine class responsibilities and interfaces.

Because of the communication diagram's format, they tend to be better suited for analysis activities. Specifically, they tend to be better suited to depict simpler interactions of a smaller number of objects.

It is also difficult to show additional descriptive information like timing, decision points, or other unstructured information that can be easily added to the notes in a sequence diagram.

Unlike a sequence diagram, a communication diagram explicitly shows the relationships among the elements. On the other hand, a communication diagram does not show time as a separate dimension, so the ordering of messages and concurrency must be determined using a numbering scheme. Sequence diagrams and communication diagrams both express interactions, and are semantically equivalent, but show them in different ways.