FINRA Rule 5310

refers to the imperative that a broker, market maker, or other agent acting on behalf of an investor is obligated to execute the investor's order in a way that is most advantageous to the investo [..]

This considers various characteristics of the financial instrument concerned: price, costs, speed and likelihood of execution and settlement, the size of the order, the possible execution venues avail [..]

The obligation of intermediaries to achieve the best possible result when executing customer orders.