precision is related to the number of significant digits in the quantity measured.
8.00 mm => precision is 0.01 mm
8.00 cm => precision is 0.01 cm = 0.1 mm
8.00 m => precision is 0.01 m = 1 cm
8.00 km => precision is 0.01 k m = 10 m
The most precise is the value 8.00 mm as it is able to specify the quantity up to the level of 0.01 mm. This will required precision instrument to do that.
