Lead Error over 300 mm
The variation over 300 mm is the most common definition. However it is also important to know whether the allowed variation is cumulative, or not. If the allowable variation is specified as "cumulative" then the permitted error over the full travel of a screw can result in a total error which is equivalent to the next lower accurancy class.
Stroke 900 mm, accuracy class P3, 3 x 300 mm would result in a total deviation of 3 x 12 µm = 36 µm. This result is equivalent to the total error of accuracy class P5 (35 µm)! However, the admissible deviation ep for such a screw in accuracy class P3 is 21 µm.