Bracket orders are designed to help limit your loss and help lock in a profit by "bracketing" any order with two opposite-side orders. A BUY order is bracketed by a high-side sell limit order and a low-side sell stop order. A SELL order is bracketed by a high-side buy stop order and a low side buy limit order.

The order quantity for the high and low side bracket orders is the same of the quantity of the original order. By default, the amount off the current price to which a bracket order is set is 1.0. This offset amount can be manually changed on the order management line for a specific order or you can modify the default offset amount using the Default Order Settings box.

You create an order to buy 100 shares of XYZ at $50.00/share and then elect to Attach Bracket Orders. TWS creates both a sell limit order for $51.00, and a sell stop order for $49.00 of the same quantity as the original order. When the original (or "parent") order executes, the two bracket orders are automatically transmitted.