Accuracy
Accuracy is how close a measurement is to the true value. If the real length is 50.00 mm and the instrument reads 50.02 mm, the error is 0.02 mm. The smaller the error, the more accurate the instrument.
Accuracy on datasheets: Maximum permissible error (MPE / Error limit)
On technical datasheets, the “Accuracy” or “Error limit” column is not the qualitative idea above but a specific figure, for example ±0.03 mm. This is the maximum permissible error (MPE), the largest error the manufacturer guarantees the instrument will not exceed under specified conditions.
It is a guaranteed upper limit, not the actual error of each measurement. The real error is usually smaller. These values are set by standards (for example DIN 862 for calipers) or by an in-house factory standard when no relevant DIN exists.
If we care about the real error of a specific instrument, it can be measured and confirmed by calibration. A calibration certificate (to ISO/IEC 17025) records the instrument’s actual deviations and confirms they stay within the declared limit.
The error limit applies to the instrument only. The real uncertainty of a measurement includes other sources as well (temperature, operator, workpiece). So the “±0.03 mm” on the datasheet is only the instrument’s contribution, not the whole uncertainty.