What It Means
The Special Pre-Open Session (SPOS) is a one-hour call auction, 9:00 AM to 10:00 AM, that runs only on an IPO's or a re-listed stock's first day of trading. Instead of live, continuous trading, every buy and sell order placed in that hour gets collected quietly and matched once, at a single price, before regular trading takes over at 10:00 AM.
How It Works
The session runs in three phases. Order collection (roughly 9:00 to 9:45) is where investors place, modify, or cancel orders with no visible order book and no running trade ticker. Price matching (roughly 9:45 to 9:55) is when the exchange takes every order collected and works out the equilibrium price, the single price that lets the largest number of shares change hands. A short buffer (9:55 to 10:00) follows before continuous trading begins.
Example
Someone holding a fresh IPO allotment places a sell order at 9:05 AM. Someone else places a similar order at 9:40 AM. Both sit in the same pool and get matched at exactly the same price once the window closes at 9:45, because the auction doesn't care when within the window an order arrived, only what's in the pool once it gets read.
Warning
Placing your order earlier in the session doesn't get you a better price. The exchange computes one price from every order together, only after the collection window closes.