Confectioners’ sugar, also known as powdered sugar or icing sugar, is a ultra fine sugar with a smooth and powdery consistency. Confectioners’ sugar is available in several degrees of fineness, from XX to 10X, and the finest is 10X. 10X is type that you are most likely to see in the market or baking supply store because it is the most widely available and the most commonly sold. Any recipe that calls for confectioners’ sugar is calling for the 10X variety unless otherwise specified. Confectioners’ sugar also typically contains a very small amount of cornstarch to prevent caking and maintain its fluffy consistency.
Confectioner’s sugar is so fine that it dissolves almost instantly in liquids, so it is most often used to making frostings or glazes in baking, where it will produce a very smooth result. It can be used in some recipes – and is occasionally called for in shortbread recipes or other baked goods where a very fine texture is desired in the finished product – but it is not frequently called for and cannot be substituted 1:1 with regular sugar in a recipe that does not call for it.