Productive efficiency
By definition, the MC curve will meet the ATC curve at its minimum point, which is the point P1 and Q1 on the diagram. At this point, producing more than Q1 would bring more costs than benefits to the firm, whereas producing less than Q1 would mean that there are more benefits than costs in producing more of the good. Hence, the point P1 and Q1 would be a point that is just right, and all the resources of the firm would be fully used in the best possible way.