There are three Auction phases: a Pre-Open Auction to determine the Opening price; a Closing auction (Pre-CSPA) to determine the closing price; and thirdly the same as the Pre-Open auction but can occur any time when a security resumes from a Trading Halt or Suspension.  During any of these auction phases, orders can be entered but they are not immediately matched (Bids and Asks orders can overlap). When the auction closes, An algorithm calculates the matching price by matching all the overlapping bids and offers.  All orders within the overlap are executed at the matching price.   See Equity Market Phase table for the full duration times.