Which UML diagram focuses on the order of messages passed between objects?

The focus of a Sequence Diagram is on the order of messages passed between objects. This type of UML diagram is specifically designed to detail how objects interact in a sequential manner over time. In a Sequence Diagram, the vertical axis typically represents time, with the various objects displayed horizontally across the top. As events occur, they are shown as arrows indicating messages sent from one object to another, along with the sequence of these interactions. Therefore, it effectively illustrates how the objects collaborate to fulfill a specific use case or functionality through the flow of messages, highlighting not just the interactions but also the timing of those messages.

In contrast, other diagrams like Class Diagrams focus more on the structure of the system, showing classes and their relationships independently of time. Collaboration Diagrams emphasize the roles of the objects in a system rather than the timing of the exchanges. Use Case Diagrams outline the interactions between users (or actors) and the system as a whole, focusing on use cases rather than the details of the object interactions over time. Thus, for understanding message order and timing between objects, the Sequence Diagram is the appropriate choice.