Order Routing
NASDAQ offers an optional routing feature that routes orders to other exchanges or to an ECN when those markets display quotes at better prices. If an order remains unfilled after being routed, it will be returned to NASDAQ or to another market with better prices. A market order that is routed to another market will be routed as a limit order.
The most common order routing option, SCAN, attempts to execute an order on the Market Center at the inside
