Implementation reminder: Nasdaq’s rollout is scheduled for today, following PSX on September 14 and NTX yesterday. Orders can now specify self-match prevention behavior and a group ID, overriding broader defaults; supported actions include cancelling the oldest or newest order, decrementing both, and disabling prevention. The optional fields apply to Enter and Replace messages across OUCH 5.0, CORE FIX and INET FIX. Nasdaq implementation notice, field details
The practical consequence is control over which liquidity survives when two strategies collide. Cancelling an old resting order sacrifices queue position; cancelling the incoming order may interrupt a hedge. Moving that choice to individual orders gives strategy developers more freedom, and makes centrally agreed defaults easier to override. This morning’s check is that the intended policy survives order replacement and that an omitted field behaves as expected—not merely that the new message parses.