Communication diagrams belong to the class of interaction diagram. They convey
the same information as sequence diagrams, but they focus on
object roles instead of the times that messages are sent. In a
sequence diagram, object roles are the vertices and messages are
the connecting links.
The object-role rectangles are labeled with either class or object
names (or both). Class names are preceded by colons ( : ).
Each message in a communication diagram has a sequence number. The
top-level message is numbered 1. Messages at the same level (sent
during the same call) have the same decimal prefix but suffixes of
1, 2, etc. according to when they occur.