Superfine sugar improves the texture of cakes by aerating the batter with its many tiny crystals and sharp edges.
Superfine sugar also dissolves well in cold and room-temperature liquids, making it useful for meringues (which weep if there's any undissolved sugar). But unlike confectioners' sugar, superfine sugar is granulated. It's also called ultrafine, instant dissolving, bar, or castor sugar, its British name.
Of all the granulated sugars, superfine has the tiniest and most uniform crystals. The tiny granulation improves the texture of cakes and other butter-and-sugar batters because the crystals' many sharp edges cut into the butter during creaming, forming many air pockets. If you can't find superfine sugar, you can make your own by grinding granulated sugar in a food processor for 30 to 40 seconds.
While shopping for sugar, keep in mind that the sugar industry has not standardized its labels, so stay alert to inconsistencies between brands.