Core terms
Queue position
Where a resting limit order stands among the orders waiting at the same price on one venue. Orders ahead of it trade first, so a touch of the price is not the same as a fill.
On US exchanges the general priority is price, then display, then time: better prices first; at the same price, displayed orders before non-displayed ones; among those, earlier orders first. A new bid at $3.20 behind 8,000 displayed shares fills only after those shares trade or cancel, or when sellers trade through the level (Illustrative). Moving or replacing an order usually sends it to the back.
Queue position is a main reason simulator fills differ from live fills. Many simulators fill a resting limit order as soon as price touches it, which adds winners that a real order at the back of the line would have missed.
Each venue keeps its own queue, so two traders bidding the same price on different venues can get very different fills. Hidden size at a price waits behind the displayed size there.
What traders should verify
For passive entries, note the displayed size ahead at your price when the order posts and whether it filled, partly filled or missed. Compare sim and live fill rates for resting limit orders separately from marketable orders.
Do not conflate
"Queue position" is trader usage; the priority rules behind it are exchange rules and differ in detail by venue and order type. A retail Level 2 does not show your exact place in line: size ahead can cancel, and size on other venues or hidden size can trade first.
Primary source
Primary sourceNasdaq Equity 4, Rule 4757 Book Processing