Order flags let you set how long the order is valid for. The typical two choices even appear on widgets such as the chart, and they are Day and GTC. You can click these on the chart to make them the default when you place an order. You can also set them with the Flags dropdown list, in the middle right of the Order Ticket window, above the Sell button.
The four flag options are as follows.
This order stays active until filled or until the trading day ends.
GTC (Good 'til Canceled):
This order will stay open until you manually close it, even into new trading days. Remember when you use this flag to avoid surprises.
Fill or Kill:
Designates a trade to be carried out immediately and entirely. If this is not possible, the order will be cancelled, even if some of the order could have been filled by the market.
Immediate of Cancel:
Similar to Fill or Kill. The difference is that any partially fulfilling the quantity is acceptable. The portion of the order that cannot be filled is then cancelled.