Those are referring to two different things. Caster sugar refers to the size of the sugar granules (it's a bit finer than granulated sugar), while cane sugar just means that it comes from sugarcane. Sugar can also come from sugar beets, in which case it'd be called beet sugar. The closest substitute for caster sugar here in North America would be superfine sugar, or you could just blitz regular granulated sugar in a food processor to make the granules finer. However it depends on the application; lots of times you'd be able to get by just fine with regular granulated sugar. The biggest difference will probably be for applications like meringues and cookies where the finer granules of castor sugar will dissolve easier.