By placing an exchange order, the customer instructs the exchange to buy or sell securities for his/her account. These orders contain precise details of the security in question: the number of units, the nominal value if any, the period of validity of the order and the price. If the specified purchase price is lower than the market price, the order cannot be executed. In the case of a sell order, the rate entered must not be higher than the exchange rate. Therefore, a price limit is usually set for the execution of the order.