A waterfall shows how signed contributions build a running total. Positive changes increase the running value and negative changes reduce it. Use it for a bridge or a set of changes with a meaningful order. A normal column chart compares independent values and doesn't show that accumulation.
A combo chart puts columns and a line on the same category axis. A second numeric axis can help when the measures use different units, but its scale needs clear labels. Sales and orders should not silently share a scale that makes the smaller quantity unreadable. Choose the category grain first, then assign the column and line measures. In these exercises, a supplied review variance includes negative contributions so you can check that the waterfall preserves the sign.