Altova website: UML Communication diagrams
Communication diagrams display the interactions i.e. message flows, between objects at run-time, and show the relationships between the interacting objects. Basically, they model the dynamic behavior of use cases.
Communication diagrams are designed in the same way as sequence diagrams, except that the notation is laid out in a different format. Message numbering is used to indicate message sequence and nesting.
UModel allows you to generate Communication diagrams from Sequence diagrams and vice versa, in one simple action see "Generating Sequence diagrams" for more information.