Castor sugar is known as superfine sugar in the United States. Because it dissolves more quickly than standard granulated sugar, it is used mainly in applications where this is desirable. It is most useful in making meringue, sweetening cut fruit and the like, where no heat is applied, and therefore the larger granules of standard granulated sugar may be undesirable. In most cases granulated sugar may be substituted, and occasionally powdered sugar is also a suitable substitute.
Castor sugar is sometimes spelled caster sugar but the Oxford English Dictionary gives preference to the former.