All operations (setting, changing, deleting, activating or deactivating etc)  with pending orders of any type can be made only outside the 10 pips range between order's rate and current market price because these orders can be at already under the supervision of the dealer.

So TradingDesk Pro 5 system prevents from placing, changing parameters or canceling of these orders inside the 10 pips range. This rule applies to trailing orders as well. This rule is called "locking rule".

Please note that after setting Limit Profit or Stop Loss order linked to Market of GTC order these Limit Profit and Stop Loss would have the opposite side to initial Market or GTC order and "locking rule" should be calculated from opposite side of the price of initial order.

For example, you are placing GTC order to buy at 1.2050. At the moment of execution of this GTC order ask-price would be 1.2050, but  bid-price would be at 1.2046. If you set Stop Loss or Limit Profit (liked to initial GTC)  orders inside 1.2050-1.2036 range or 1.2060-1.2050 range it will not be accepted by system. In this case GTC order will be executed, but Stop Loss or Limit Profit order placed inside aforesaid ranges will not be set to opened position.

This locking rule can be downwarded without preliminary notice: 

Note that "locking rule" acts for real accounts only. These rules for demo and contest accounts can differ.