In a Sequence diagram, the workflow is represented by the sequence of Messages down the diagram. Messages near the top of the diagram are passed before Messages lower down the diagram.

Consider the following diagram.

Message 1 is earlier than Message 2. However, in a complex diagram, or when representing finely timed operations or parallel processing, this might not be apparent.  You can reinforce the sequence using a General Ordering arrow.

Click on the Message arrow. A small arrow displays at the source anchor point.

Click on this arrow and drag it to the start of the next Message in sequence (Message 2 in the example). The General Ordering arrow displays, indicating that the second Message follows the first.

The General Ordering arrow is exaggerated in the above figure. You would normally have the arrow running almost horizontal across the diagram.

You can have more than one General Ordering arrow issuing from or targeting a Message, if necessary.