The figures below show a maximal effort flow volume curves (red) and sub-maximal effort curves starting with the poorest effort (green) left to the maximal effort -- bottom right (red). It is unlikely any flow-volume curve will exceed the maximal effort curve (red) and when curves with repeatable size and shapes are seen, usually this indicates a maximal effort has occurred.
Notice in the above curves and the curves shown on the left, the flows continue to increase from green curve, to blue curve, to black curve and then red curve, until the maximal effort curve (red) is achieved. The size and shapes of the curves will be the same once this maximal effort has been obtained. The values will also be repeatable.