The diameter of a steel rod is given as 56.47±0.02 mm. This means that 56.47 mm is the actual diameter of the rod and ±0.02 mm is the least count of the instrument. It means the least count of the device that is used to measure the diameter of a given steel rod is 0.02mm.
The uncertainty number of the device is 2. This is unable to measure the length beyond the two digits after decimal in mm scale.