It is an optional add-on feature to a limit or market order. It is used by a trader when he/she intends to not disclose or indicate the complete quantity of an order. For any given order the minimum quantity which the client needs to disclose is 10% of the order.
Example: Suppose, Mr. X wants to buy 10,000 stock. If he intends to not disclose his order size to the market then he can set the disclosed quantity to 1,000 which is 10% of 10,000.