Confectioners’, powdered, or icing sugar is granulated sugar that has been beaten, crushed, trampled, stomped, trodden, squashed, and ground into a fine powder. Because it tends to form clumps, confectioners’ sugar is augmented with about 3 percent cornstarch to keep it loose and flowing.
Confectioners’ sugar is ground to different degrees of fineness – the most common of which are XXX, XXXX, and 10X – where the grains are finer as the number of Xs increases.
Because confectioners’ sugar dissolves almost instantly, it is generally used in dishes and recipes that don’t require cooking, such as icings, sauces, and some candies. You can cook with confectioners’ sugar, but very few people do. First, it is about twice as expensive by weight as granulated sugar. Then you have to use 1-3/4 cups for every cup of granulated sugar, making it nearly twice as expensive again.
On top of that, you have to take into account that 3 percent cornstarch, which will provide some degree of thickening as it cooks. There may be times when you want that little bit of thickening, such as a cooked sauce. But there are certainly some recipes where the texture would not be benefited by the additional cornstarch.