There are various definitions of what the opening range means, but the most common is that it refers to the gap between the low and high price of the first 30 minutes of trading.
Trading a specific number of shares or a fixed dollar amount makes little sense when trading the opening range breakout. Instead, you must alter the position size to match the range of your reference candle.
The use of a fixed risk and fixed target simplifies trade management. You place your stop-loss one tick below the opening range breakout low and leave it there.
When you use a trailing stop, you move the stop one tick below the low of the most recent candle with each completed candle. The process works well with high-momentum stocks that see fast price changes, and in some circumstances, the price continues to trend up throughout the trading day.
Some momentum stocks rise by 20%, 50%, or more on the first trading day. If you hit a home run, using a fixed risk per transaction with no defined target can result in significant returns.