Not enough rep to comment, but wanted to correct a comment by @John Birckhead. The bias instability is the minimum on the Allan Variance curve, which represents low-frequency noise (similar to drift) that cannot be corrected by averaging more samples (see article). It is how stable the bias is during a run. The "initial bias" refers to the change in bias from run to run (after turning off the device) and I believe that is what Bosch refers to as the zero-rate offset. It's confusing as both parameters are reported in the native units of deg/s.
Bias Repeatability (Turn-on to Turn-on Bias):
For each powerup of the IMU, the initial bias is different. This is due to a number of effects,including change in the physical properties of the IMU and initial conditions of signal processing. A very repeatable bias allows for better "tuning" of IMU parameters by the INS to quickly reach a good estimate of the bias. A high variability in the turn-on to turn-on bias, leads to a longer and more difficult estimation period (convergence) for every start up.
Bias Stability (In-run Bias):
While the IMU is powered on, the initial bias changes over time. This change in bias is often related to temperature, time and/or mechanical stress on the system. In the case of light based gyroscopes (Fibre Optic Gyro (FOG)/Ring Laser
yro (RLG)), the optical length increases or decreases with the change in the physical properties of the IMU. Often, IMUs are manufactured with temperature compensation, increasing the stability of the measurements. An INS filter constantly estimates the bias by making use of external sources of information
(GNSS, DMI, barometer). The estimated bias value is removed from the IMU measurements before using them in the mechanization. The process of estimating the bias is more effective when stable. The effect of bias stability
can be observed directly in the outage performance.