In a sequence diagram, what do vertical arrows represent?

Vertical arrows in a sequence diagram represent messages exchanged between objects or components in a system. In this context, a message can indicate a request, response, or data being passed from one object to another. The direction of the arrow shows the flow of communication, meaning who is sending the message and who is receiving it.

In sequence diagrams, the vertical axis typically represents time, with messages being displayed in chronological order down the diagram. The use of arrows makes it easy to visualize the interactions and the sequence of operations among various entities, such as objects or actors, within a specific scenario. This graphical representation aids in understanding how components collaborate to fulfill a particular use case or process.

The other choices, while related to elements of system design, do not accurately describe the specific function of vertical arrows in the context of a sequence diagram. Objects refer to the entities involved, commands may pertain to specific actions within the system but do not universally represent messages, and events describe occurrences that might trigger actions but are not depicted by vertical arrows in this format.