How do you obtain covariance from manufacturer datasheet?

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I am looking through the imu driver code (microstrain_3dmgx2_imu) to understand how I can write a driver for my own IMU.

I notice the parameters for orientation/angular velocity/linear acceleration stdev were derived from the manufacturer datasheet ("The default value, derived from the manufacturer datasheet, is very conservative.")

What I would like to know is, how is this derivation done? For example, if the "Accelerometer nonlinearity" is 0.2%, and "Accelerometer bias stability" 0.010 g, how do I get the variance?