A question about a recipe: Celli Ripieni (Grape jam dumplings)

The name caster or castor sugar is used in the UK and Australia - it's simply a fine, white sugar (but not as fine as confectioners or icing sugar), most often called for in baking and desserts like meringues. I believe in the US it's called "superfine" sugar.

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I don't purchase superfine sugar or castor sugar. I take regular sugar and put it in my food processor. My daughter who bakes a lot swears by Baker's Sugar. I still take the food processor method. Below is the definition of castor sugar.
From Ochef.com:

"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.

Because of its fineness, it dissolves more quickly than regular white sugar, and so is especially useful in meringues and cold liquids. It is not as fine as confectioner’s sugar, which has been crushed mechanically (and generally mixed with a little starch to keep it from clumping)."