When a set of objects is being counted, the result is exact.
But when something is being measured, the result is rarely
exact. It depends on the scale of the instrument being used
and the purpose of the measurement.
For example, international 100 m athletics races are timed
to the nearest 1/100 th of a second. If a pin is measured
using a ruler and its length is given as 2.3 cm, then its
length is not exactly 2.3 cm but will be between 2.25 cm and
2.35 cm. When a measurement is expressed using a given unit,
the greatest possible error is half of that unit.