Sand or the other aggregates added to sealer must fall within a set of particle sizes, neither too coarse nor too fine. This is ascertained by the sieve analysis which means that sand has been sifted through a set of screens with varying mesh sizes meaning the openings in the screen. Percent retained means how much of 100 grams of sand was retained on the screen and % passing is how much passed through the screen. Use your manufacturers recommended grades.