The acceptance region plot is used in variables acceptance sampling. When the upper and lower specifications are known, and the standard deviation is unknown, the acceptance region plot lets you see the region of sample means and sample standard deviations for which you will accept a lot.
This plot is useful in illustrating your sample requirements. As the sample standard deviation increases and approaches the maximum, the mean needs to be on target for you to accept a shipment. If the process variation is tight and the standard deviation is small, the mean can vary between the specification limits.