What does Bias Repeatability tell me in an IMU's spec sheet?
No, there is not a uniform specification.
I've worked with electromechanical gyros, and those did have a true turn-on to turn-on bias change.  So there would be one bias, then you'd cycle power and there'd be another, etc.
If you're buying enough, you can always ask the manufacturer for clarification.  Sometimes this works if you're a student, too.  Otherwise, just accept the fact you're comparing apples to oranges and do your best.


It is typical that if gyro bias is truly that important you either find a way to calibrate it on the fly*, or you go buy a bigger, more expensive gyro.

* If you're doing sensor fusion, you can make this part of your state estimation.