No answer yet.
I have heard that one pulse typically causes a rotation of 1.8 degree in stepper motor.
That is, 200 pulses for a complete rotation. If gear ratio is R, and pitch of the lead screw is 10 mm, this will cause a displacement of 10/(200R) per pulse, which is equal to 0.05/R mm. To ensure an accuracy of 0.001 mm, R should be minimum 50. On a servo, similar thing might be happening.
How accurate is my information? I would appreciate correct / more detailed information.