This is an order to be executed at the market price when the market closes. Institutions frequently use this method to buy or sell large numbers of shares, e.g., when a stock is to be added or deleted from an index and the institution must buy or sell the stock for an index fund. These orders are also used on day trades in order to close a position at the end of the session regardless of the price. It can also be used when you want to exit a position that day, but do not want to cut off a stock that is running during the day-place a market on close order and your trade will be executed at the closing price. As with all market orders, there is risk in that the stock that was rising and hit a target you would be happy at selling at, but then falls right before the close.