Saw this question in Oliver Lehmann 75 questions.

What is defined by control limits?

1. A measuring instrument solely used to describe process capability. The process is found capable if the ±3 sigma range is exceeded by not more than 0.3% of a tested sample lot.

2. The limits of the six sigma area on either side of a control chart to plot measured values; data found outside the area are out of specification and can lead to rejection of an entire batch.