What are all the digits that are known in a measurement plus the last digit that is estimated?

In a measurement, the known digits are those that have been accurately determined through the measuring process, while the last digit is an estimate based on the precision of the instrument used. This last digit reflects the uncertainty inherent in the measurement. Together, these digits provide a complete representation of the measurement's value, indicating both the certainty and the uncertainty involved. For example, in a measurement of 12.3, "12" is known, and "3" is the estimated digit.