for nyse open cross, this is from its website:
The Core Open Auction serves as a bridge between the Early Trading Session and the Core Trading Session. The execution of the Core Open Auction is the first trade of the Core Trading Session. Limit Orders eligible for the Early Trading Session only will be cancelled back after the Core Open Auction.
Core Trading Session eligible Limit Orders, Market Orders and Auction-Only Orders will participate in the Core Open Auction. Beginning at 9:29 a.m. ET, requests to cancel or cancel and replace Market on Open (MOO) and Limit on Open (LOO) Orders will be rejected. Beginning five seconds before the Core Open Auction, new MOO and LOO Orders will be rejected and new Limit Orders (that are not eligible to trade in the Early Trading Session) and Market Orders will be accepted, but will be eligible to participate in the Core Open Auction only to offset the imbalance that is remaining after all orders entered before that time period are allocated in the Core Open Auction.