Courtesy of the defendants, a roster that includes Mastercard Inc. and Visa Inc. as well as a number of big-name banks, millions of mostly small and mid-size merchants will now split a $6-billion pot to compensate for damages they allege they suffered through paying artificially high discount fees for card transactions. Left unsettled, however, and looming ominously on the horizon, is the other part of the matter. That's because some of the nation's biggest merchants, including Home Depot Inc., 7-Eleven Inc., and Target Corp., opted out of the money-damages settlement and focused instead on something that's always been part of the case but cuts much closer to the bone for the payment networks: the networks' mandates and restrictions surrounding card acceptance.