Setback

Setback is the relative positions of the two wheels on an axle.

Setback can be given in mm or as an angle i.e. degrees and minutes.

Sign convention
Negative setback is the right wheel back (relative to the left)
Positive setback is the left wheel back (relative to the right)

Which wheel
Setback will not tell you which wheel is in the correct position only the relative position. So is that wheel forward or the other wheel back?
Measuring wheelbase or caster can help.

Tolerances
Interestingly very few manufactures give a tolerance, so the value should be used as a guide in that if you measure a high set back, there may then be something else which will measure out side the tolerance. One example of this would be the caster angle. If one front wheel has been knocked back giving it high set back then the caster angle would also measure low, possibly even out of tolerance on this side.