This dialog appears when you click the Downstream Trace button. Clicking in the drawing pane when this dialog is active will set the start point for the downstream trace.

Ignore Depression Depths Less Than field: If the value of the field is the default value 0, the downstream trace may stop at a local lower point. If the field value is set to larger than zero, the downstream trace will ignore local depressions with depth less than the value and continue the downstream trace to a lower point.