Rounding a number with the accuracy specified by the number of decimal places given in the rounded value. So e = 2.71828… = 2.718 to three decimal places and = 2.72 to two decimal places. $\sqrt{86.56}$ = 9.30076… is 9.30 correct to two decimal places. Where a number of quantities are being measured and added or subtracted, using values correct to the same number of decimal places ensures that they have the same degree of accuracy. However, if the units are changed, for example between centimetres and metres, then the accuracy of the measurements will be different if the same number of decimal places is used in the measurements.