What element is essential for marking the end of a single flow of control?

The element that is essential for marking the end of a single flow of control is the Flow Final Node. In modeling, particularly within activity diagrams, the Flow Final Node serves as a definitive indicator that a specific flow of actions has concluded. It ensures that once the flow reaches this node, it cannot continue further, effectively terminating that particular path of execution.

The Flow Final Node is important because it signifies that no further actions or transitions will occur along that flow. This is different from other nodes in that they might represent alternative paths or the convergence of flows, but the Flow Final Node specifically marks a clean end to a flow, indicating that the sequence has completed.

In contrast, while the Activity Final Node also marks the end of an entire activity diagram, it can represent the termination of all flows within the diagram, rather than just a singular flow of control. Understanding these distinctions is crucial for accurately identifying how flows are controlled and terminated within different modeling contexts.