Quote from Sintra:
Regulatory imbalance occurs when normal imbalance is above 50.000. You can trade against a regulatory imbalance after 15:45 but not against the normal one. That is a normal limit on close order.
You do have a closing offset order. I think it is possible to enter this one after 21:45 regardless of an imbalance. This order has the lowest priority for a specialist and comes in last. It only trades if it offsetts the imbalance.
Hope this helps
Quote from newdog:
Why is regulatory imbalance only for close and not for open on NYSE?
Quote from Sintra:
Don't know for sure but i think that has to do with liquidity for the specialist. He could use all the offsetting to trade before market open. In the close his trading/position building is done before the close and therefor huge liquidity so 50,000 shares is no problem.