Confectioners’ sugars are ground to a fine powder and mixed with a small amount of starch (about 3%) to prevent caking. They are classified by coarseness or fineness.
10X is the finest sugar. It gives the smoothest texture in icings.
6X is slightly coarser in texture than 10X. For this reason, it is less likely to form lumps or to dissolve in moisture. It is used mostly for dusting the tops of desserts.
Coarser types (XXX and XX) are used for dusting and whenever 6X or 10X are too fine.
Confectioners’ sugar is also known as icing sugar because of its importance in making many kinds of icing.