I must add that icing sugar is adulterated with cornstarch to prevent clumping, so it cannot be substituted for caster sugar, a.k.a. castor sugar, superfine sugar, or berry sugar, which do not contain cornstarch.
But the plot thickens, for the good sneff informs me that in Australia there is one product called icing sugar, which is pure powdered sugar used for dusting and the like, and another product called icing mixture that contains cornstarch and is used for making icing (frosting). What we get here in Canada labelled icing sugar is a very very fine powder, very white - indeed not unlike cornstarch in appearance and texture. I've never seen a product labelled icing mixture here.