Most orders that you place on the exchange are by default GTD orders. A GTD order is valid till the end of the trading day. When the trading for the day closes, the order is automatically cancelled. GTD orders become relevant when either your market orders do not get executed due to poor liquidity or your Limit orders do not get executed due to unfavourable price. In both these cases, the GTD order will be held in the system till the end of the trading day.

There is also another kind of an order which is IOC (Immediate or Cancel) order where the order is executed only if it can execute the moment it is placed. Else, the order stands automatically cancelled. This type of order is more popular among institutional investors and large proprietary traders.