A servo that repeats to a tight number and a servo that positions accurately are not the same claim. Backlash is where the two separate.

Put two dial indicators on a gear train — one on the input, one on the output — rock the shaft and read the difference. That gap is the distance between where your controller believes the shaft is and where it actually sits.
Why it matters more than the datasheet:
1. It passes acceptance testing. Backlash is present at commissioning and grows with wear. A machine that qualified in month one can start rejecting parts in month eight with no fault code.
2. Software cannot remove it. You can compensate a known offset. You cannot compensate a gap that opens in one direction and closes in the other.
3. It is set by manufacturing, not by spec. Gear material, tooth profile, preload, and how the housing supports the bearing under load. Two suppliers quoting the same torque can ship very different backlash behaviour.
Where it hurts most: robot joints doing pick-and-place, AGV steering that hunts at low speed, and any axis that reverses direction under load.
We build for low backlash and quote the achievable figure per application rather than publishing one headline number — because the number depends on your duty cycle and your gear ratio.