Linearity is the maximum difference in output, at any measured value within the specified range, when the value is approached first when increasing and then decreasing pressure.
For example, consider a linearity value of ±0.03%FS .
The raw output of transducers exhibits an "S" shaped curve. During calibration, the technician adjusts the electronic output to make the "S" curve as straight as possible. Straight still has some dips and rises. Those dips and rises are no greater than ±0.03%FS from a perfect straight line.
To get the measurements they see what the output is when they approach a point, ex. 3"WC, starting from zero. Then they would see what the output is when they approach a point from the opposite end of the pressure range.