RE:TriAct ExchangeTD Securities ( and other discount brokers ) employs a sophisticated routing system that dynamically assesses multiple marketplaces and distributes your order to the marketplace that, based on its analysis, is most likely to obtain the highest-quality execution available. In addition, TD Securities continually monitors its execution quality to make improvements. TD Securities employs multiple metrics, including price, speed, liquidity and opportunities for price improvement, to ensure that the order flow for TD Direct Investing, TD Wealth Private Investment Advice and TD Securities is directed to marketplaces that provide you with the best possible execution. TriAct is part of those Alternative Trading System ( ATS ) . Dark Pools are ATSs which do not display orders prior to execution. That's why sometimes you want to buy more shares that's showing on the ask and you get filled right away...