S.F. stands for "significant figures".
they are used to simplify large complicated numbers.tis is done as it does not make any significant difference while doing the calculation.as if you do calculation with .00234*some number .then if we neglect last value the accuracy will will reduce to 4 digit. means we get correct answer for four decimal place.
For example 12981342 could be written to 4 sf as 12980000.