These days it's pretty rare to see the large firms actually use their market maker identification on listed stocks. Typically, the market makers will route their order flow to the ECNs or alternative trading systems rather than show their hand by identifying themselves. On OTC stocks it's a different story, but even then, the size on the ECNs often dwarfs the size visible on the market makers' books. Ironfist, as was mentioned before, you're not seeing an entire level II, only what is on ARCA's orderbook. In fact, ARCA only does something like 10% of the daily volume of NYSE-listed stocks, so you're really only scratching the surface of what's out there. On a more full level II, you would want to see, in addition to ARCA's order book, Island/NASDAQ's order book, the NYSE order book and outstanding orders from less popular venues such as Direct Edge, the Chicago Stock Exchange, the National Stock Exchange and the Boston Exchange.