https://www.nasdaq.com/articles/routing-201%3A-some-of-the-choices-an-algo-makes-in-the-life-of-an-order-2019-11-14

Phil Mackintosh – “One benefit of exchange mid-point orders is that Exchange mid-point orders should execute faster, especially when that exchange has shares on the NBBO. That’s because sellers on the exchange will trade with the mid-point order as they try to hit lit bids. Data shows some exchanges have quite a lot of mid-point liquidity.” … “With over 50 different places to source liquidity, including around 30 dark pools, it’s tricky for an algorithm to know where to route first, especially for dark pool orders.” … “