It is a transparent system that matches customer orders (e.g. bids and offers) on a 'price time priority' basis.
The CLOB is by definition fully transparent, real-time, anonymous and low cost in execution.
Such a book was proposed as a centralised database of limit orders by the U.S. Securities and Exchange Commission in 2000.
The customer may only "hit the bid" (sell to the highest bidder) or "lift the offer" (buy from the cheapest seller).
The customer is prohibited from stepping inside the bid/ask spread and thereby reducing its execution fees.