Concept
Exchange Order Flags
Exchange Order Flags, also known as reduce-only, post-only, iceberg, are Risk, Sizing & Exits concepts. First implementations are in the build queue: the write-up leads, the indicators follow.
What are Exchange Order Flags?
Exchange order flags are modifiers attached to an order that change how the matching engine is allowed to handle it, without changing the order's basic type. The three most consequential for traders are reduce-only, which permits the order to decrease an existing position but never open or flip one; post-only, which cancels or reprices the order if it would execute immediately, ensuring it rests in the book as a maker order; and iceberg (or display-quantity) flags, which show only a slice of the full size in the visible book, replenishing as each slice fills.
The flags exist because raw order types leave gaps between intent and execution. A trader closing a position with a plain limit order can accidentally end up short if the position was already closed by a stop elsewhere; reduce-only makes that impossible at the engine level. On venues with maker-taker fee schedules, the difference between posting liquidity and taking it changes the economics of a strategy, and post-only converts a fee assumption into an enforced property. Iceberg flags respond to information leakage: displaying full size invites the market to trade around it, so the flag lets size work passively while revealing little.
These behaviors matter twice over. For execution, they are risk and cost controls that belong in any systematic workflow, especially on crypto derivatives venues where reduce-only and post-only are standard order-ticket options. For analysis, they leave footprints: repeated refills at one price hint at hidden size, which is the basis of iceberg detection as a tape-reading technique. Availability and exact semantics vary by venue, so the same flag name can behave differently across exchanges, and some markets do not offer native icebergs at all.
How traders use it
- Reduce-only on every exit order: systematic traders flag all stops and targets reduce-only so that overlapping exit orders can never net into an unintended opposite position, a cheap safeguard against a class of automation bugs.
- Post-only for fee-sensitive strategies: market-making and rebate-dependent systems submit post-only so a stale price never turns an intended maker order into a taker fill; the cost is occasional missed fills when the book moves through the price.
- Icebergs for working size: traders with orders large relative to typical book depth display a fraction of true size to reduce information leakage, accepting that queue position resets on each refill and that sophisticated participants may still infer the hidden quantity.
- Reading flags in the tape: repeated executions at a price whose displayed size keeps replenishing in the depth-of-market display suggest hidden interest, which some order-flow traders treat as a level worth respecting.
- Combining flags with cost planning: post-only versus taker execution is one of the levers in execution cost modeling, since fee differences and fill probability trade off against each other.
Exchange order flags vs related order concepts
Order Types: Order types (market, limit, stop) define how an order seeks execution. Flags are constraints layered on top of a type, restricting what the engine may do with it.
Order Constructs: Constructs like brackets and OCO groups link multiple orders into one logical position plan. Flags operate at the single-order level and are often applied to the legs of such constructs.
Iceberg Detection: The analytical mirror image of the iceberg flag: techniques for spotting hidden size from refill behavior in the book and prints on the tape.
Related concepts · Orders & execution
Concept family
Risk, Sizing & Exits
37 concepts mapped · 37 in the Library
Exchange Order Flags FAQ
What happens if a post-only order would cross the spread?
Depending on the venue it is either rejected outright or repriced to the best passive price. Either way it never executes as a taker, which is the point of the flag.
Can a reduce-only order increase my position?
No. If the resting reduce-only quantity exceeds the remaining position, the excess is canceled or resized by the engine, so the order can only shrink exposure toward flat.
Are iceberg orders visible to other traders?
The hidden portion is not displayed, but the refill pattern is often detectable: fills at one price repeatedly exceeding displayed size are a classic tell that experienced tape readers watch for.
Do all exchanges support these flags?
No. Crypto derivatives venues commonly support all three, traditional futures and equity venues vary, and semantics differ by matching engine, so behavior should be verified per venue before automation relies on it.
Do order flags cost anything to use?
Not directly, but each embeds a trade-off: post-only risks missed fills, icebergs sacrifice queue priority on refills, and reduce-only orders may be resized under you if position size changes.
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