When setting a pending order (Stop, Limit or Stop-Limit) on the Invest/ISA platform, you can see at the bottom of the order’s tab that you have two options for setting an Expiration - End of Day and GTC.
With the 'End of Day' option, the pending order will be automatically cancelled by our platform if it's not executed by the end of the trading day on which it is placed.
Conversely, GTC stands for 'Good til cancelled', and unlike the orders that have the expiration option set as 'End of Day', your 'GTC' orders will remain pending until they are either executed by our platform or cancelled from your side.
Note: GTC orders will not remain pending indefinitely though. A GTC order will expire on the last trading day of the quarter following the quarter in which it was placed. For example, when you create a GTC order in January, if its target price is not reached by the last trading day of the following June, it will be cancelled by the system.
With the 'End of Day' option, the pending order will be automatically cancelled by our platform if it's not executed by the end of the trading day. 
Note: As of 20/09/2021, accounts registered under Trading 212 Markets Ltd. will have the option to choose between 'End of Day' or 'GTC'. Already existing 'End-of-week' orders will expire at the end of the following week if their price is not reached.