Control Limits - are calculated as three standard deviations away from the mean on either side of the mean. Also known as process limits, these are derived basis the process data and represents the area within which natural variation can be expected (common cause variation). Any observation/data point outside the control limits is a reason for investigation and will always have an assignable cause (special cause)
Upper Control Limit - three standard deviations above mean
Lower Control Limit - three standard deviations below mean
An application oriented question on the topic along with responses can be seen below. The best answer was provided by Venugopal R on 10th November 2017.