In the testing phase, which UML diagram is essential for illustrating interactions?

The Sequence Diagram is crucial during the testing phase as it provides a detailed view of the interactions between different elements in a system over time. It allows testers to visualize how various components communicate with each other, showcasing the order of messages exchanged and the interactions that occur in a given scenario. This is particularly important for identifying potential issues or misunderstandings in how components should work together, making it easier to validate that the system behaves as expected under different conditions.

By focusing on the time aspect of interactions and the sequence in which events occur, the Sequence Diagram helps ensure that all processes are correctly followed and that dynamic behavior matches the specifications. This is beneficial during testing because it can reveal any discrepancies between the expected and actual behavior of the system that might not be visible in more static representations like Class or Component Diagrams.