Imbalance-Only And Auction-Only Orders
Two special order types built specifically for call auctions — auction-only orders that never trade in the continuous market, and imbalance-only orders that can only add to the side of the book that already has too little interest — exist to let liquidity providers help clear the print without taking on open-ended risk.
Prerequisites: Tie-Breaks: Minimum Surplus And Reference Price, Time In Force: Day, GTC, IOC And FOK
Most order types were designed for continuous trading, where a resting order can be hit at any moment. Auctions are different: everything settles at one price, once, at a specific instant, and the exchanges have built order types that only make sense in that context.
Auction-only orders
An auction-only order (sometimes called "on-open" or "on-close" without a limit-order twin) participates exclusively in the auction and is cancelled if it doesn't execute there — it never spills into the continuous session before or after. A trader using it is saying "I want this filled at whatever price the auction determines, or not at all right now," which matters for anyone building a portfolio-level benchmark around the official open or close print: they don't want a stray partial fill happening in the regular book at a different price.
Imbalance-only orders
An imbalance-only (IO) order is more specialized still: it can only add liquidity to whichever side of the book currently has the smaller cumulative quantity — the side needed to reduce the imbalance. If the auction shows a buy imbalance, a resting IO sell order can participate; an IO buy order sitting on the same (already oversized) buy side simply cannot trade, no matter its price. This is deliberate: IO orders exist to let market makers and liquidity providers step in and help the auction clear without ever risking that their order makes an already one-sided imbalance worse.
Worked example: who trades, who doesn't
The pre-close book shows a buy imbalance of 150,000 shares at the indicative price. Four resting orders are queued:
| Order | Side | Type | Participates? |
|---|---|---|---|
| A | Sell | Auction-only, limit 50.00 | Yes — auction-only orders trade if marketable at the print |
| B | Buy | Imbalance-only | No — buy side already has the surplus, IO buy can't add to it |
| C | Sell | Imbalance-only | Yes — sell side is the deficit side, IO sell is exactly what's needed |
| D | Buy | Regular limit order (also valid in continuous market) | Yes, if marketable — behaves like any normal auction-eligible order |
Order B is the instructive one: it's a perfectly valid, marketable buy order in every other sense, but the imbalance-only restriction blocks it specifically because letting it trade would push the buy side even further out of balance — the opposite of what an IO order is meant to do.
What this means in practice
These order types give liquidity providers a way to lean into an auction imbalance without contributing to it: an IO order guarantees you can only ever make the print fairer, never more lopsided. Anyone estimating auction depth from the order-type mix needs to know that IO interest on the surplus side is dormant and won't show up in the eventual matched volume no matter how the price moves.
Auction-only orders trade solely in the auction or not at all; imbalance-only orders go further and can only add to whichever side currently has less quantity, so they help absorb an imbalance but can never make one worse.
Related concepts
Practice in interviews
Further reading
- Nasdaq Rulebook, IEX and NYSE order type specifications