For cheapskates:
On a side note, you can make really strong vanilla sugar by saving your used vanilla beans (after you make pastry cream for example.) Wash them well and dry them on top of your oven or some place warm for a week or so. When they are done, they should be extremely hard (like coffee bean hard.) Pulse them in a coffee bean grinder and pass it through a very fine sifter and add the powder to some sugar. Use where ever you use regular sugar (but you want vanilla flavor.)
I think here in North America, the equivalent of caster sugar is “berry sugar”, which is inbetween icing sugar and granulated.
I get confused on some cooking terms, because some Canadian sources use American terms, and some use British terms.
And don’t get me started on the British custom of weighing everything, instead of going by volume … I had to get a kitchen scale just to try some of the receipes I’m experimenting with. Fortunately, I was able to obtain a very accurate one second-hand, that passed inspection from Canada Weights and Measures in 1966 - says so, right on the inpsection label. So I know my 2 oz is really 2 oz. 
Luckily, it’s really simple to make, and the homemade stuff is far superior to anything you can buy in a jar.
(Too good, in fact - the first time I made it, I think I went through the whole batch on toast within a 36-hour period.)
Is there some reason why, in a recipe that calls for vanilla sugar, you couldn’t just use regular sugar and add a bit of vanilla to the recipe? 'Cause that’s what I’d do…which is probably why just about every time I try to bake I regret it.
I don’t see anywhere that implies that “superfine” means finer than powdered. I would assume it just means finer than regular granulated.
I noticed a couple of years ago that supermarkets out here on the left coast have begun carrying “baker’s sugar,” which is just a finer version of granulated sugar. 'twould be quite easy to manufacture in a blender or processer though. I’m sure I’ve seen Alton do it…
I hear you can make vanilla sugar by splitting a vanilla bean lengthwise, scraping out the seeds, then storing it in a container with sugar at the bottom of the Marianas Trench, for 20 minutes.
(In 1960.)
What’s berry sugar?
Berry sugar is superfine sugar. Domino’s is the brand I’m most familiar with in the US. It melts more quickly than granulated. It’s frequently used to coat berries and other small fruits in a hard sugar glaze.
Berry sugar is a form of sugar that does not require splitting a vanilla bean lengthwise, scraping out the seeds, then storing it in an airtight container for a couple of weeks.
Seriously, the differences between all these various types of sugars are set out on this page: FoodSubs: Sugars: Ingredient Substitutions and Ingredient Synonyms
We just need someone to come along and make a joke about splitting a 1920s-style death ray lengthways…