Quote Rule

It's a stock market thing. And it exists as an exchange rule.

Think: something from NASDAQ, the NYSE, and its brethren. It requires market makers to be fully disclosative of stock price quotations for any listed security, when that set of offers represents more than one percent of the aggregate trading volume of that security.

Why does this rule exist? Because if a large transaction is in the process of happening, like a founder dumping her shares, or an LBO artist coming in to take over the company, that information is directly applicable and relevant to current and potential owners of the security. They have a right to know about the freight train heading at them, or leaving the station, and so the quote rule ended up being a Thing.