Not an ECN. The MPID NQPX referrs to the new US equity stock exchange PSX owned and operated by NASDAQ OMX. See http://www.nasdaqtrader.com/Trader.aspx?id=nasdaqpsx NASDAQ re-activated the equity exchange that they got when they bought PHLX, but using their own technology. PSX is different because it uses price/pro-rata-size allocation instead of price-time. Pro-rata allocation means that resting orders at the same price level get to fill part of the incoming order according to their relative sizes (instead of oldest order first)