Protection is a narrow privilege. Only the best bid and best offer on an automated exchange qualify — not depth behind the top, not hidden size, not manual quotes, and not alternative-trading-system interest.
This is why the nbbo can look tight while real size is expensive: a 100-share protected offer legally constrains routing, but it does not mean 100,000 shares are available there.
Example: an exchange shows 100 shares offered at 45.00 and 30,000 at 45.03. Only the 100 at 45.00 is protected. A buyer of 30,000 must take that 100 first, then may fill the rest at 45.03 or worse without breaching the order-protection-rule.
Related: nbbo, order-protection-rule, trade-through, intermarket-sweep-order