Resolved Question

Castor or caster sugar is the name of a very fine sugar in Britain, so named because the grains are small enough to fit though a sugar "caster" or sprinkler. It is sold as "superfine" sugar in the United States.

Powdered sugar, also known as confectioner's sugar or icing sugar, is granulated sugar that has been ground into a very fine powder.

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 on baked goods to add a light sweetness and subtle decoration.

Caster or castor sugar has a larger particle size, up to approximately half that of granulated sugar.

Hope that helps!