CLOB. System of aggregate limit orders on all securities received by specialists, market makers, and electronic trading systems. The central limit order book can fall into one of two designations: "soft" or "hard." The difference between the two is that a "hard" CLOB executes limit orders immediately and a "soft" CLOB executes limit orders as they are facilitated. This system has been advocated by the SEC but opposed by most major exchanges due to the fact that the exchanges could potentially lose volumes of trades.