Caster sugar is slightly finer then 'granulated sugar' typically sold here in the USA. Buying a specific type of sugar you likely won't use very often doesn't seem worth it. For the type of dessert being described here, I don't really think it makes that much of a difference. For something like a cake frosting, you would use confectioner's sugar and wouldn't substitute since the result would be too gritty. For most other desserts involving baking, I would use the regular sugar you likely already have at home.