O que é National best bid and offer?

Example 9.5 (Three venues)

Venue X shows $10.00 \times 10.02$ (300 by 500 shares), venue Y $10.01 \times 10.02$ (200 by 100), venue Z $10.01 \times 10.03$ (400 by 900). The NBBO is $10.01 \times 10.02$, 600 by 600: the bid from Y and Z, the offer from X and Y. A broker that fills a client’s market purchase at 10.03 while X and Y offer at 10.02 has traded through two protected quotations. If a fourth venue bids 10.015 for 60 shares, nothing changes: an odd lot is not protected. The NBBO is the benchmark of every execution-quality statistic of Chapter 10; it is also, as we now see, not a single thing.