What the messages' from and to name it by. Unique in the layer.
OptionallabelWhat it is called when it is announced. Defaults to id.
OptionalkindWhat sort of participant it is, as the diagram draws it: actor,
participant, database, boundary and the like. Announced with
its name.
OptionalgroupThe named box the diagram groups it in, when there is one.
One participant of a sequence diagram: a lifeline.