The IRS is investigating whether federal tax withholding and reporting requirements were met for gambling profits distributed to 600 members of the tribe from 2006 to 2009.

FORBES: IRS Wins Fight With Native American Tribe

Other dark pools may show symbol and side, send single or market-wide IOIs or set criteria such as minimum participation requirements so that when an IOI message is distributed either to a single contra-side or a marketplace, there are ways of "reading" into the order information.

FORBES: Don't Be Afraid Of The Dark Pools