UML22 — Interactions

In the description of the Graphic Paths for a Communication Diagram I can
find no mention of what the lines between the Lifelines correspond to -
although I did find this in the description of Message: "On Communication
Diagrams, the Messages are decorated by a small arrow along the connector
close to the Message
name and sequence number in the direction of the Message." I assume this
means that the lines correspond to a Connector model element.

The Graphic Paths section should be updated to include this information and
justification added as to why a Connector is needed in order for Messages to
be shown between two lifelines on a Communication Diagram (this seems an
overly tight constraint to me).

see above