In a message sequence chart it is often important to segment the process into multiple logical parts. You can use the ‘---’ element to draw a horizontal line acorss the chart with some text, e.g., to summarize what have been achieved so far.

Similar to this, using the ‘...’ element can express the passage of time by making the vertical lines dotted.

Sometimes one merely wants to add some text to a chart. In that case the empty element can be used either like ‘: text;’.