Alternative control limits. American and Japanese practice uses 3σ control charts. That is, the control limits are three standard deviations on either side of the mean. When the statistic being plotted has a Normal distribution, the probability of a point outside the limits is about 0.003 (or about 0.0015 in each direction) by the 68-95-99.7 rule.
European practice uses control limits placed so that the probability of a point outside the limits when in control is 0.001 in each direction. For a Normally distributed statistic, how many standard deviations on either side of the mean do these alternative control limits lie?