What is Protected quotation, trade-through, order protection rule?

A protected quotation is the best bid or best offer of an exchange that is displayed, automated and immediately accessible. A trade-through is the execution of a trade at a price inferior to a protected quotation displayed elsewhere: a purchase above another venue’s protected offer, or a sale below its protected bid. The order protection rule (Rule 611) requires every trading centre to have policies reasonably designed to prevent trade-throughs.