Arrows with a solid head represent a message sent from one actor to another. The arrow points from sender to receiver. Arrows with a line head represent the return value of a message. The arrow points from the receiver of the earlier message to the sender. Intersections between arrows and dotted lines are meaningless.
Vertical layout represents time.
Horizontal layout has no formal meaning. Since right-pointing arrows look better, actors that initiate a scenario tend to appear leftward of actors they send messages to. Note that sequence diagrams are often used for more concrete design, and that actors and messages are often objects and object methods. They provide value for this document in that they give a clearly ordered message layout. The actors and messages in the sequence diagrams below are more properly roles in a scenario and actions associated with that scenario.