At The Close Order is an order that stipulates that the security is to be bought or sold only at the close of the market, or as close to the close as is reasonable, or not at all.
Applying "At The Close Order" to Securities Exams:
An investor may wish to buy or sell a security at the close of the market’s regular hours. If the order is not executed on the close the order will be canceled. The investor may place the order as a market on close or limit on close order. If the security is listed on NASDAQ the order will be part of the closing cross.