Icing Sugar is made from normal sugar but it's made into a different form of sugar than normal granulated sugar is.
The icing sugar is different from normal granulated sugar when it comes to cooking and baking.
When foods are baked the sugar that is used is powdered sugar or confectioners sugar which is also called icing sugar in some places.
So powdered sugar is the same as confectioners sugar and icing sugar is the same as confectioners sugar and powdered sugar.
But regular granulated sugar that you use in tea, coffee etc is different.
But in reality the icing sugar as well as confectioners sugar and powdered sugar are all made from regular granulated sugar.
But when you are baking goods you should use the icing sugar, powdered sugar or confectioners sugar.
It's basically all the same sugar but is different in the way it's made and used.