National Best Bid and Offer (NBBO) is a regulation by the United States Securities and Exchange Commission that requires brokers to execute customer trades at the best available ask price when buying securities, and the best available bid price when selling securities.
To qualify for consideration, a quote must be issued by the primary (NYSE) specialist and be BBO-eligible, that is, be eligible for inclusion in the Financial Industry Regulatory Authority Best Bid and Offer calculation.
References
National best bid and offer Wikipedia(Text) CC BY-SA