This guide explains order execution, matching rules, order validity, slippage, partial fills, liquidity impact, order placement steps, and key execution risks.
Basic Concept of Order Execution
Order execution refers to the process in which, after a trader submits a buy or sell instruction, a broker, trading service provider, or exchange trading system looks for a counterparty according to market rules and completes the transaction. An order is not necessarily executed at the expected price after it is submitted. The actual outcome depends on market quotes, order book depth, liquidity, order type, validity period, and the rules of the trading venue.
In a trading context, placing an order expresses trading intent, while execution means the order has been accepted by the market and matched. Traders may request immediate execution, or they may require execution under specific price, time, or quantity conditions. Different order settings affect execution speed, execution price, and the probability of non-execution.
Which Participants Are Involved in Order Execution?
From submission to execution, an order usually passes through multiple stages. The specific structure differs across markets, but the basic logic is similar.
Trader: submits a buy or sell instruction and sets the trading instrument, quantity, direction, price condition, and validity period.
Broker or trading service provider: receives client orders and, according to relevant rules, routes them to an exchange, market maker, liquidity provider, or internal execution system.
Exchange or trading system: matches buy and sell orders according to matching rules such as price priority and time priority.
Market maker or liquidity provider: in some markets, continuously provides bid and ask prices to improve tradability.
Clearing and settlement institution: handles the transfer of funds and assets after execution. In stock markets, common settlement cycles are T+1 or T+2, subject to local market rules.
Brokers are usually expected to seek a reasonable execution outcome in accordance with applicable regulatory requirements and their own order execution policy. However, market prices change continuously, and orders cannot always be executed exactly at the quote seen by the trader, the price set by the trader, or the planned quantity.
Order Book and Matching Mechanism
An order book is a list that records buy orders and sell orders in the market. The prices buyers are willing to pay form the bid side, while the prices sellers are willing to accept form the ask side. The difference between the best bid and the best ask in the order book is called the spread. A smaller spread usually indicates lower transaction costs; a wider spread indicates a more obvious disagreement between buyers and sellers, or insufficient liquidity.
For example, a trader wants to buy 100 shares of ABC Company at 250 pence per share. If there is a seller in the order book willing to sell at least 100 shares at 250 pence, the order can be executed at that price. If only 40 shares are available for sale at 250 pence, the remaining 60 shares must either wait for new sell orders or, under market order rules, continue to execute at higher ask price levels.
Why Price Touching Does Not Equal Full Execution
When price touches a certain level, it only means that the price or quote appeared in the market. It does not mean that all orders can be fully executed. Execution also requires sufficient counterparty order quantity.
Price condition: the order price must meet the conditions set by the trader, such as market price, limit price, or trigger price.
Quantity condition: the counterparty order quantity must be sufficient to satisfy all or part of the order demand.
Queue priority: multiple orders may exist at the same price level, and trading systems usually process them according to time priority.
Market speed: when prices move quickly, an order may face a new quote environment after being triggered.
Order permissions: different accounts, trading venues, and products may support different order types.
Execution Mechanism of Market Orders
A market order is an order that requires execution as quickly as possible at the currently available market price. It emphasizes execution speed rather than precise control over the execution price. A buy market order seeks the current ask quote, while a sell market order seeks the current bid quote.
If market liquidity is sufficient, a market order can usually be executed quickly. If the order size is large or the quantity at the best quote is insufficient, the order may consume multiple price levels in sequence, resulting in different execution prices. The difference between these execution prices and the trader’s expected price is commonly called slippage.
Layered Execution Example of a Market Order
Assume trader Tom wants to buy 500 shares of ZXY plc. He sees a screen quote of 175.00 pence per share and submits a buy market order. If the order book shows available sell quantities of 150 shares at 175.00 pence, 200 shares at 175.50 pence, and 150 shares at 176.00 pence, the market order will be executed layer by layer against the available ask side.
First, 150 shares are bought at 175.00 pence, because this is the current best ask price.
The remaining 350 shares continue to seek the next available ask level.
Then, 200 shares are bought at 175.50 pence, leaving 150 shares still to execute.
Finally, 150 shares are bought at 176.00 pence, completing the full 500-share order.
The final execution result is 150 shares at 175.00 pence, 200 shares at 175.50 pence, and 150 shares at 176.00 pence.
This example shows that the quote displayed for a market order may not cover the full order quantity. If the quantity at the best quote is insufficient, the order will execute at the next price level, and the final average execution price may be higher than the initially displayed 175.00 pence.
Partial Execution and Inventory Execution
Partial execution means that only part of an order quantity is executed, while the remaining quantity continues to rest in the market, is cancelled, or is handled according to the order rules. Partial execution is common with limit orders, low-liquidity instruments, large orders, and fast-moving markets.
In some market structures, a broker may act as a market maker or internal liquidity source and execute against clients from its own inventory. Inventory execution is not the same as direct order book matching. The specific arrangement depends on the broker’s role, product type, client agreement, and regulatory requirements. Traders need to read the order execution policy to understand whether orders may be routed to an exchange, market maker, liquidity pool, or internal execution system.
Execution Characteristics Under Insufficient Liquidity
Limited executable quantity: a single price level cannot satisfy the full order quantity.
Wider bid-ask spread: the difference between a trader’s buying cost and selling recovery price increases.
More obvious slippage: market orders may sweep through multiple quote levels.
Slower execution speed: limit orders may remain pending for a long time without enough counterparties.
Increased gap risk: obvious intervals may appear between adjacent execution prices.
Major stocks, major foreign exchange currency pairs, and products linked to large stock indices usually have relatively high liquidity, but this does not mean there is no slippage. Major economic data releases, corporate earnings announcements, market open and close periods, periods around holidays, and market stress conditions may all change order execution conditions.
Definition and Mechanism of Order Validity
Order validity refers to the time range during which an order remains executable in the market. It answers a key question: if the order is not executed immediately, should the system keep it waiting, or cancel it under specific conditions?
Validity settings can help avoid unexpected execution after market conditions have changed. For example, a trader sets a gold buy condition today, but the price does not reach that condition until one year later. By then, the original macro background, account funds, trading plan, and risk tolerance may have changed. Through validity settings, traders can reduce execution uncertainty caused by long-standing orders.
| Item Name | Key Parameters | Applicable Scenarios | Main Risks |
|---|---|---|---|
| Day Order | Valid until the end of the trading day; commonly called GFD or Day Order | Suitable for orders intended to execute only under the day’s market conditions | If not executed before the close, the order automatically expires and may miss the next day’s price condition |
| Good Till Cancelled Order | Remains valid until executed, cancelled, or reaching the platform’s maximum validity period; commonly called GTC | Suitable for limit or stop-type orders that wait for a specific price over a longer period | May still execute after the market background changes; different brokers may set 30-day to 180-day validity limits |
| Immediate Or Cancel Order | The unexecuted portion is cancelled immediately; commonly called IOC | Suitable when the trader accepts only immediate executable quantity and does not want the remaining order to stay in the market | May be only partially executed, with the remaining quantity cancelled |
| Fill Or Kill Order | Must execute immediately in full, or the entire order is cancelled; commonly called FOK | Suitable when partial execution is not acceptable and the full quantity must be executed at the same time | Execution conditions are strict, and the entire order may fail if liquidity is insufficient |
Meanings of GTC, GFD, GTD, IOC, and FOK
A good till cancelled (GTC) order remains valid until it is executed, cancelled by the client, or reaches the broker’s system limit. Different platforms may set maximum retention periods for GTC orders, such as 30 days, 90 days, or 180 days, so traders should not assume that all GTC orders can exist indefinitely.
A good for day (GFD) order remains valid until the end of trading on the day it is placed. If it is not executed before the market close, the order is usually cancelled automatically. Traders need to confirm the regular trading session, pre-market and after-hours arrangements, and holiday rules of the market being traded.
A good till date (GTD) order allows the trader to set a specific expiration date or time. If it is not executed before expiration, the order automatically becomes invalid. GTD is suitable for limiting an order plan to a specific research period, event window, or trading plan.
An immediate or cancel (IOC) order requires the system to immediately execute the executable portion and cancel the remaining quantity that cannot be executed immediately. IOC allows partial execution, so it is suitable for situations where the trader accepts a partial fill but does not want the remaining order to remain exposed in the market.
A fill or kill (FOK) order requires the entire quantity to be executed immediately and in full; otherwise, the whole order is cancelled. FOK does not accept partial execution, so it is stricter than IOC. If a trader submits a 500-share FOK buy order while only 300 shares are available under the qualifying price condition, the order will not be executed.
Price Deviation in Order Execution
Price deviation in order execution mainly comes from changes in liquidity, market volatility, quote delays, and order size. Market orders are the most likely to experience price deviation because they do not set an upper or lower limit on the execution price. Limit orders can control the worst acceptable execution price, but they may fail to execute.
Why Market Orders May Execute at Less Favorable Prices
The execution logic of a market order is to find available counterparties as quickly as possible, not to wait for a specified price. If the order quantity submitted by the trader exceeds the quantity available at the best quote, the system continues to search for the next price level. A buy market order may execute at higher ask prices, while a sell market order may execute at lower bid prices.
The larger the order size, the more likely it is to consume multiple quote levels.
The faster quotes update, the more likely the difference between the price seen by the trader and the system execution price may widen.
The thinner the market depth, the more easily even a small order may move the execution price.
During market opens, closes, and major news releases, the size of slippage may widen.
Some brokers set maximum slippage, price tolerance, or requote mechanisms, subject to the platform’s specific rules.
A limit order can avoid execution at a price beyond the set boundary. For example, a buy limit order will not execute above the limit price, and a sell limit order will not execute below the limit price. However, the cost of using a limit order is a lower probability of execution. After price reaches or passes the limit price, if executable quantity is insufficient or the order is behind in the queue, it may still be only partially executed or not executed.
Operational Process of Order Execution
Understanding the order execution process helps traders clarify their objectives and limits before setting an order. An order is not a single button, but a combination of conditions.
Confirm the trading instrument, including stocks, foreign exchange, stock indices, futures, options, or CFDs.
Confirm the trading direction and distinguish among buying to open, selling to open, buying to close, and selling to close.
Confirm the order type, such as market order, limit order, stop order, or stop-limit order.
Confirm the order quantity and assess whether the order size may exceed the currently visible liquidity.
Confirm the price condition and determine whether immediate execution or price boundary is more important.
Confirm the validity period and choose conditions such as GFD, GTC, GTD, IOC, or FOK.
Check estimated costs, including spreads, commissions, financing costs, stamp duty, or exchange fees.
After submitting the order, check its status and distinguish among filled, partially filled, pending, cancelled, and rejected.
Common Meanings of Order Status
Submitted: the order has been sent to the broker or trading system but has not yet been matched.
Filled: the full order quantity has been executed.
Partially filled: part of the order quantity has been executed, while the remaining quantity continues to wait or is cancelled according to the order rules.
Pending: the order is still waiting for price conditions, quantity conditions, or counterparty quotes.
Cancelled: the order has been cancelled by the trader, system, or validity rule.
Rejected: the order was not accepted due to price limits, account permissions, insufficient margin, market closure, or product rule mismatch.
Risk Control Points in Order Execution
Order types can improve execution management, but they cannot eliminate market risk. Traders need to understand price risk, liquidity risk, technical risk, and rule risk at the same time. Especially in leveraged products, the outcome of order execution directly affects account equity, margin usage, and the probability of forced liquidation.
Price risk: market prices may change between order submission and execution.
Liquidity risk: insufficient buyers or sellers in the market may cause partial execution or wider slippage.
Validity risk: long-standing orders may execute after the trading background has changed, while short-term orders may expire and miss the price condition.
Gap risk: the market may move past a price range, causing the order to fail to execute at the expected level.
Technical risk: network latency, platform outages, quote suspensions, and exchange circuit breaker mechanisms may affect order execution.
Contract rule risk: futures, options, and CFDs also involve contract multipliers, expiration dates, margin ratios, and forced liquidation rules.
For margin products such as CFDs, leverage ratios also require attention. In some regulated markets, common leverage limits for retail clients are 20:1 for major stock index CFDs, 30:1 for major foreign exchange currency pair CFDs, and 5:1 for single-stock CFDs. Leverage amplifies the impact of price changes on account equity, so both execution price and executed quantity may affect actual risk exposure.
Questions Related to Order Execution
Why might an order not be executed immediately after submission?
Whether an order is executed depends on price conditions, whether there are enough counterparties in the market, queue priority, and validity rules. A limit order may remain pending because the price has not been reached or quantity is insufficient. Although a market order is usually executed faster, its execution price may deviate.
Why can a market order have multiple execution prices?
If the quantity at the best quote level is insufficient to satisfy the entire order, a market order will continue executing at the next available price level. Therefore, one buy market order may be partially executed at 175.00 pence and partially at 175.50 pence or 176.00 pence.
What is the difference between IOC and FOK orders?
An IOC order requires immediate execution of the executable portion, with the unexecuted remainder cancelled, so partial execution is allowed. A FOK order requires immediate full execution, otherwise the entire order is cancelled, so partial execution is not allowed.
Is a GTC order valid forever?
GTC means the order remains valid until it is executed or cancelled, but many brokers and trading venues set a maximum validity period, such as 30 days, 90 days, or 180 days. Traders should follow the specific platform rules.
Why might an order be only partially filled after price touches the limit?
A limit price is only a price boundary. Execution also requires sufficient counterparty order quantity. If the executable quantity at the same price level is insufficient, or if the order is behind in the queue, the order may be only partially filled, while the remaining portion continues to wait or is cancelled according to validity rules.
What is the purpose of setting order validity?
Order validity limits how long an order remains executable in the market. It can reduce the possibility of old orders being unexpectedly executed after market conditions have changed, and it can also help traders control whether partial execution is allowed, whether immediate execution is required, or whether the order should expire after the market close.
