A confirmation can carry three prices for one instruction. A market order to buy 1,000 shares of XYZ at $3.00 per share can have 500 shares execute at $3.00 and the other 500 execute at a higher price, because parts of a large order run out of liquidity at one price. The condition that produces the split is size against the quote: price quotes are only for a specific number of shares, so an order larger than the quoted size cannot all trade there. What happens to the shares that did not fill is set by the instruction you attached, not by the market.

Why one order becomes several executions

The SEC's order-types bulletin, updated 18 August 2026, treats this as ordinary, because a quote is two prices and two sizes. The bid is the highest price a buyer will pay to buy a specified number of shares, and the ask is the lowest price at which a seller will sell. The specified number is the part that matters here. An order for more shares than sit at the best price takes what is there, then looks for the next price.

Three mechanics produce the pieces. Size is the first: the price at which a market order executes often deviates from the last-traded price because of demand and the prices of available liquidity at the time of execution. Price limits are the second, because a buy limit order can only be executed at the limit price or lower, so the walk up the book stops at your limit instead of continuing. Routing is the third, since a broker evaluates competing markets, market makers and electronic communications networks when seeking the best execution reasonably available, and one order can be worked across more than one of them.

None of this is an error in the order. It is the order meeting the size of the market.

Example: 500 shares filled in three pieces

An example, and a mechanism rather than a strategy: the $20.00 limit is illustrative and the three fill prices are fictional, since no public figure fixes a specific order book. A buy limit order for 500 shares at $20.00 goes to the market and comes back in three executions: 100 shares at $19.85, 100 shares at $19.95, and 200 shares at $20.00.

The arithmetic runs straight down the confirmation. 100 times $19.85 is $1,985. 100 times $19.95 is $1,995. 200 times $20.00 is $4,000. Total shares filled: 400. Total cost: $7,980. Average price: $7,980 divided by 400, or $19.95 a share. Every one of those prices is at or below the $20.00 limit, which is what a buy limit order at the limit price or lower requires.

That leaves 100 shares unfilled out of the 500 requested. The position is 400 shares, not 500. The average cost is $19.95, not $20.00. Both numbers matter later, because the cost basis of the shares you actually hold is what turns a fill price into a net result.

The instruction decides what happens to the remainder

Those 100 shares have four possible fates, and you chose one before the order was sent.

Left as a plain day order, they stay open at $20.00 until the session ends, then cancel. Marked good-til-canceled, they stay open past the close within the firm's own time limit on GTC orders. Marked immediate-or-cancel, they never wait at all: any portion of the order that cannot be filled immediately is canceled, so the record would show 400 shares filled and 100 canceled at once.

The last two instructions would have changed the whole example. A fill-or-kill order must be executed immediately in its entirety, and otherwise the entire order is canceled, with no partial execution allowed — under that instruction the 400 shares never fill either, and the position is zero. An all-or-none order must also be executed in its entirety or not at all, but it remains active until it is executed or canceled, so nothing fills now and the full 500 keeps waiting for a moment when 500 shares are available at $20.00 or better.

Choosing between them is what the time and fill conditions on an order are for, and the choice between certainty of price and certainty of execution is the same one that sits behind a market order and a limit order. A partial fill is the visible half of the same trade-off that makes a fill price differ from the quote.

Source references

Primary sources used for this guide. Follow the links for the original scope, definitions and conditions.