Castor (or caster) Sugar is fine powder sugar, so named because the grains are small enough to fit though a sugar "caster" or sprinkler. It is slightly coarser than icing sugar and at least 950 g/kg of Castor Sugar shall pass through a sieve having a mesh aperture size of 0.599 mm. It is called "superfine sugar" in U.S.A. & "berry sugar" in British Columbia. The fine sugar crystals of Castor Sugar incorporate more air than other types of sugar.
When used in baking, caster sugar gives more volume to the mixture, resulting in extra-light cakes. Castor Sugar is also perfect for light and crisp, melt-in-mouth puddings and for sprinkling over cereal, doughnuts and fresh fruit. Because of its fineness, it dissolves more quickly than regular white sugar, and so is especially useful in meringues and cold liquids.
Icing sugar also known as Powdered sugar and confectioners sugar is very fine sugar. When intended for home use, it typically contains a small amount of anti-caking agent.
In industrial food production, it is used where a quick dissolving sugar is required. Domestically, it is principally used to make icing or frosting and other cake decorations. It is often lightly dusted onto baked goods to add a light sweetness and subtle decoration.