Caster/Castor sugar is finer than granulated and creams, mixes and melts quicker. Sometimes called superfine sugar. We use caster for making cakes and meringues. It isn’t as fine as icing (confectioner) sugar. I didn’t realise it wasn’t a universal sugar. Sometimes when I am a bit short of caster sugar for a recipe I put granulated in my processor and pulse it a few times - maybe if you can’t source caster you could try this.