As a trader using Level II execution programs, keep the following information in mind always.
Market makers are able to use any or all of the ECNs to place bids and offers.
However, most generally use INCA, and Island. And a very important point to remember is that market makers can and do post incorrect quotes to attempt to sway traders in their decision to either buy or sell a stock. More about this later. It is an independent system and is accessible via direct referencing. Many level II providers such as Kubertrading using the Kubertrading system allow for direct access or direct linking to the island system, and the island book which shows the complete depth of bid and ask at the various pricing levels of a stock as posted to Island.