hard · Market Microstructure

An exchange offers a 'hide-not-slide' style repricing for displayed orders. The NBBO is bid $15.00 / ask $15.01 (one-tick spread). A trader posts a displayed buy limit at $15.01 (locking the market). The venue reprices it to a non-displayed ranked price of $15.00 but ranks it as if at $15.01 for priority purposes, sliding it back to $15.01 only if the locking offer leaves. A second trader then posts a displayed buy at $15.00 a moment later.

When the $15.01 offer is cancelled (unlocking the book), which buy order trades first against the next incoming marketable sell, and why?

  1. The first trader's repriced order executes first because it retains hidden priority at the $15.01-equivalent rank from its original timestamp and slides up to $15.01 ahead of the later $15.00 displayed bid.
  2. The second trader's $15.00 displayed order executes first because displayed liquidity always outranks any hidden or repriced order regardless of timestamp.
  3. Both orders are now at $15.00 once unlocked, so allocation is by displayed-size pro-rata between them irrespective of arrival order.
  4. The first trader's order is cancelled on unlock because hide-not-slide orders expire when the lock that triggered them resolves, leaving the $15.00 order to trade first.

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