In a number derived from a measurement the last significant digit to the right inherently expresses an uncertainty. For example if you are sure that your low resolution quadrupole type mass spectrometer can deliver accurate measurements to a tenth of a mass unit then you would be justified in reporting masses to a tenth of a mass unit. For example if one measured a mass of 110.1 u this number would contain four significant figures with the last digit expressing the uncertainty . The uncertainty would be plus or minus 0.1 u. Even if the instrument is capable of reporting 10 digits passed the decimal point one should only report the significant digits.
Errors can arise in calculations if insignificant figures are used in a calculation.
If a number resulting from a measurement is used in a calculation that involves multiplication or division all significant figures should be carried through the calculation and then the result should be rounded at the end of the calculation to reflect the term used in the calculation with the fewest significant figures. For example 10.4 X 5.0 should be reported as 52 and not 52.0. If the calculation involves addition and subtraction a different rule applies, one should preserve common decimal places of the numbers involved. All non-zero numbers are significant. All zeros between significant numbers are significant, f or example the number 1002 has 4 significant figures.