IEX Group’s plans to bring its “broker priority” market structure into the light have stirred up a debate about exactly how this system works and whether anyone is disadvantaged.

Just to recap: IEX, hero of Michael Lewis's "Flash Boys," is a dark pool marketed as a fairer place to buy and sell stocks, but it always had plans to become an exchange at some point. It is now raising money and preparing its application with US regulators to become a registered exchange.