When creating a limit order, you will need to designate a target spot price, which is the spot price at which your order will execute. Buy limit orders will execute when spot price falls to, or below the target spot price that you designate. Sell limit orders will execute when spot price rises to, or above the target spot price that you designate.
For example, if you create a buy limit order for 10 ounces of U.S. Gold, with a target spot price of $1,200, the order will execute, and buy 10 ounce of U.S. Gold when spot falls to $1,200, or lower. If you create a sell limit order for 10 ounces of U.S. Gold with a target spot price of $2,500 the order will execute, and liquidate 10 ounces of U.S. Gold when spot price rises to, or above $2,500.