Order execution is the completion of all or part of an instruction to buy or sell an instrument, creating a trade at an agreed price and quantity. Each completed portion is a fill. For a quant strategy, fills determine the acquired exposure and trading cost: submitting an order, receiving an acknowledgement and observing a quote do not establish what the strategy actually bought.
Execution can occur through an exchange's matching system or through another trading arrangement, such as a dealer. The example below uses a synthetic order book. Its rules and available quantities are stipulated; it is not a live quote or a claim about every venue.
One instruction, two fills
A buyer submits order O-70 for 70 shares of fictional stock XYZ. Assume an eligible market order arrives at a frozen book offering 50 shares at USD 101 and 100 shares at USD 102. There are no fees, hidden orders, competing messages, routing changes or binding venue controls. Assume the matching system produces the two separately identified fills below.
| Event | New executed shares | Cumulative executed shares | Shares still executable | New gross purchase obligation, USD |
|---|---|---|---|---|
| Order accepted, before any fill | 0 | 0 | 70 | 0 |
| Fill E-1: 50 shares at USD 101 | 50 | 50 | 20 | 5,050 |
| Fill E-2: 20 shares at USD 102 | 20 | 70 | 0 | 2,040 |
There is one order and two economic executions. The first fill is a partial fill; the second completes the requested quantity. The SEC's order-types bulletin explains why parts of a stock market order can execute at different prices as available liquidity changes or is consumed.
Original synthetic event ledger. Rows show event order, not elapsed time. Prices are USD per share; purchase obligations exclude fees. No settlement date or universal venue message sequence is assumed.
The gross purchase amount is USD. The quantity-weighted average execution price is
Neither 101, the best offer before matching, nor 102, the last fill price, is the average price paid for all shares. The simple average of the two fill prices, 101.5, also gives the wrong answer because it weights each fill equally. This is the average of this order's fills, not automatically the market's volume-weighted average price over a benchmark interval. If nothing fills, the average execution price is undefined.
A report is evidence to interpret
A message named “execution report” need not announce a new trade. In FIX's order-state specification,
ExecType explains why a report was sent, while OrdStatus describes the order's current state. Reports can acknowledge acceptance, provide status, announce a fill or communicate a correction. A separate acknowledgement before the first fill is not guaranteed by every workflow.For the example, reconcile the fill identities and economic terms with cumulative executed quantities of 50, then 70 shares. Adding those cumulative values would incorrectly book 120 shares. A repeated delivery of E-1 also adds no new position. Two received reports might describe two fills, one fill delivered twice, or acceptance followed by one fill.
Preserve source, account, order lineage and the identifier's documented scope when matching records. An execution identifier is not necessarily a globally unique trade key. Nasdaq's historical RASH FIX DROP specification, for example, explicitly describes a self-cross whose buy and sell orders share an
ExecID. Reconciliation needs the relevant side and order context, not only a bare identifier. Use the applicable broker or venue contract for current field semantics.A trade blotter preserves the resulting execution ledger. Record execution time separately from message receipt time: delayed evidence does not move the trade to the moment the trading system happened to receive it.
Cancellation, correction and settlement change different things
An order cancellation targets the unfilled remainder. A request does not itself confirm cancellation; a fill may occur while the request is being processed. It does not erase a completed trade. A trade cancellation or bust instead reverses a previously reported execution, while a trade correction amends its terms. Retain the reference chain so that the current ledger and its history can both be reconstructed. FIX documents these as distinct events.
In the uncorrected example, trade-date inventory increases by 70 shares, and the signed gross cash obligation is −USD 7,090 for the buyer. That obligation does not prove that cash and securities have already moved. Trade settlement completes the applicable delivery and payment process. The SEC's settlement bulletin distinguishes that transfer from the trade itself. The timing depends on the instrument and market; no settlement cycle is assumed here.
For the operational next step, SelfMadeQuant's Live Execution Monitoring follows confirmed fills, outstanding orders, replayed messages and cancellation races before a replacement order can safely use the remaining quantity. Its preview is public; the full lesson requires membership.
Sources
- FIX Trading Community, Order State Changes, FIX Latest as of EP284, November 2023, pp. 4–5 and sections B and J — report purpose, order state, cancellation races and execution corrections/cancellations.
- SEC, Understanding Order Types — Investor Bulletin, updated 18 August 2026 — stock orders can receive executions at several prices.
- Nasdaq, FIX DROP RASH Format — ETMF, updated 5 March 2015, §2.3 — historical example of report fields and execution-identifier scope, not a statement that this old interface is a current universal contract.
- SEC, New “T+1” Settlement Cycle — What Investors Need To Know, 27 March 2024 — settlement as transfer of securities and cash. Primary sources checked 2026-10-27.
- Rishi K. Narang, Inside the Black Box: A Simple Guide to Quantitative and High-Frequency Trading, 2nd edition, Wiley, 2013, Chapter 7, pp. 120–122 and 129–130; Chapter 14, p. 245 — partial execution, liquidity-taking, trading infrastructure and an order filling across price levels. The event ledger is original.