It stands for post no preference order plus; an order (a PNP order) representing a limit order to buy or sell a security that is to be filled in whole or in part on an exchange (a market center). The portion not executed is posted and parked in the order book without routing any part of it to another market center. The order, if marketable against the national best price on either side (NBBO) but not executable on the exchange, would be represented in the exchange’s order book by re-pricing the order.

However, if the order or a portion thereof would otherwise be locking or crossing the national best price, it would automatically be re-priced to be one minimum price valuation (MPV) above the best bid (for sell orders) or one MPV below the best offer (for buy orders), in order to avoid locking or crossing the best price. In which case, the re-priced bid or offer is added to the exchange’s disseminated quote. When re-priced, the PNP plus order is re-ranked at the new price.

For each and every change in the best price, the order would continue to be re-priced and re-ranked, until the best price moves terminating any locking or crossing of the original price of the PNP plus order against the best price. The PNP plus order would then automatically be re-priced back to its original limit price and re-ranked in the consolidated order book. The PNP plus order will not be re-priced if the order is locked or crossed by another market center.