This is the equivalent spherical diameter that has the same specific surface area per unit volume as that of the entire particle distribution. It is NOT the median value of the distrtibution, sometimes called the d50 value. The Sauter mean is invariably significantly less than the d50 value hence designs based on d50 will assume more rapid settling leading to under-designed thickener areas. This is one reason why systems with particles in sometimes do not perform according to their design.
Allowable entries are within the range 1 to 1000 micron.