An order book shows orders at a particular moment

An electronic limit order book displays buyers’ and sellers’ prices and quantities. The highest displayed bid and lowest ask form the inside market; their difference is the quoted bid–ask spread. That spread describes the gap at the best quotes, not the total cost of every possible trade. A larger order may consume size at several price levels, and displayed orders can execute or be canceled before the screen updates.

A limit order restricts a trade to a specified price or better, but reaching that price does not guarantee a fill. A market order seeks immediate execution but does not guarantee its price. SEC order-type guidance makes that distinction explicit. “Displayed size” is interest shown in the book; “executed volume” is quantity actually traded. They are different observations, not interchangeable measures of liquidity.

Hypothetical example: displayed size is not an execution promise

Consider an illustrative snapshot, not market data: the best bid is $49.98 for 500 shares and the best ask is $50.02 for 400 shares. The quoted spread is $0.04 and the midpoint is $50.00. At that instant, a 250-share market buy might match within the displayed 400 shares if that size remains when the order arrives and the venue and routing conditions allow it. The snapshot cannot tell us the full price for a 600-share order. It does not guarantee that the 400 shares will still be there; the remaining 200 could reach the next ask, but that next price and its quantity are not specified here.

This example is not an observation or forecast of an actual fill. SEC guidance on execution explains that an order takes time to reach a broker and execution venue, prices can change quickly, and quotes apply to a specific number of shares. A claim about executable liquidity therefore needs the instrument, venue, data timestamp, order size and type, and routing context. One screenshot cannot establish liquidity for an entire market.

What an algorithmic order or cancellation can establish

Algorithms may submit, amend, or cancel orders. A cancellation records that a displayed order is no longer resting in the book; by itself it does not prove the trader’s intent, manipulation, or a market-wide withdrawal of liquidity. To investigate a rise in cancellations, compare bid and ask depth, completed trades, spread, quote duration, trade prices, and volatility on a common clock. Different screen refresh rates can obscure the order of cancellations and executions, so check event timestamps and which venues the data include. This is an interpretation checklist, not an official test for a strategy or a legal violation.

In the United States, SEC Rule 15c3-5 concerns risk controls at broker-dealers with market access. The SEC Division of Trading and Markets FAQ describes pre-set credit or capital thresholds and rejection of erroneous orders outside price or size parameters; it says the controls cover computer-generated orders as well. The FAQ presents staff interpretations, not a rule text approved by the Commission. Its scope is market-access broker-dealers; it does not impose the same requirement on every algorithm user worldwide. Read the SEC FAQ for its limits.

What this explanation cannot tell you

Investor.gov describes stock liquidity in terms of buying or selling shares without substantially affecting price. That is broader than the best spread or the size on one screen. Trading can occur across venues or through processes that do not appear in a given display, and market conditions and broker routing affect execution. All numbers here are hypothetical. They do not promise an investment result or execution price and are not advice about whether to place a particular order.

Sources