With a stop market order, once the mark price hits the stop price that you specified, the stop market order becomes a market order. The market order is then matched with the best available offers in the order book.
Because a market order is executed at the best available price using orders in the book, there is a risk that the order is executed at an average price above or below your stop trigger price. While a stop market order is useful in protecting profits or limiting losses, a fast-moving market can affect the average fill price of a stop market order
To command greater control over the average fill price with a stop order, the use of stop limit orders are recommended.
A stop limit order allows you to set a specified price (the limit price) at which the order is filled. The stop limit order is then executed at the limit price (or better) once the stop price is reached. The drawback of a stop limit order is that if the market changes direction quickly, your stop limit order may not be fully executed. If the market price does not reach the limit price, then the stop limit order will not be executed at all.