Within 0.002 in (0.05 mm), or 0.5%, whichever is greater.

The tolerance reads two ways because the meaningful limit shifts with gauge length: the absolute figure governs short lengths, the percentage governs long ones.

The accuracy comes from a servo motor indexing a lead screw between marks, so each step matches the one before it. Since elongation is calculated from these marks, this figure sets the floor for how good that result can be.