In a calibration procedure, the test accuracy ratio (TAR) is the ratio of the accuracy tolerance of the unit under calibration to the accuracy tolerance of the calibration standard used.
In a calibration procedure, the test uncertainty ratio (TUR) is the ratio of the accuracy tolerance of the unit under calibration to the uncertainty of the calibration standard used.
We commonly hear about using a TAR ratio of 4 to 1, which means that the reference standard is 4 times more accurate than the device under test (DUT). I.e. the accuracy specification of the reference standard should be 4 times better (or smaller) than the one of DUT.
See more at What is calibration