A limit order book shows the number of unexecuted limit (or stop) orders resting on the exchange at each price increment. According to this unsourced quote from Cerco, the full order book which market makers have access to, contains "large orders or clusters of orders away from the current market" I assume he is referring to Limit order data or limit order book that is inaccessible to retail traders. I contacted the CME and asked why this data on limit orders is only available to market makers. The rep. I spoke with insisted no one has access to this information, including market makers since 'it could be used to manipulate the market'. His statement conflicts with cercos description of the market maker order book. I'd like to know who is correct (link the source).