Corn flour is a binding agent or to be used to thicken the sauces & gravy. Like we use it in Chinese cooking or to make pakodas crispy we use little of cornflour. Custard powder too is made from cornflour but with flavouring agents. It is mixed with milk and to be set in fridge. Or custard powder can be used in making cakes etc.
You can make different types of custard by adding corn flour to milk and flavourings