Known as “caster” sugar in much of the rest of the English-speaking world, the only real difference between this and standard table sugar is the particle size (about 0.2 millimeters). The smaller size means it has more surface area, so many bakers prefer it for creaming applications, where sugar is beaten with butter to create lots and lots of tiny holes (which will go on to become gas- and steam-trapping bubbles). Since smaller crystals melt faster, it’s often preferred for mousses and meringues. Added to cake batters it disperses in smaller pockets and can create a lighter crumb.
Superfine sugar isn’t widely available in many places, including the States. However it can be made faily easily buy processing table sugar in a food processor for about 90 seconds. Don’t do that too often though, or you’ll dull your processor blades!