Each stock is listed on an exchange. Typically NYSE or NASDAQ. Most ETFs and ETNs are on ARCA. These are the primary exchange for that stock or ETF/ETN. Those exchanges have an opening and closing auction to reach a fair price at or near 9:30 am ET or 4 pm ET. This is where most of the volume goes at that time. IF you send your order to another exchange or ECN, you have little protection at that time and will be filled based on available booked orders. https://www.nyse.com/publicdocs/nyse/markets/nyse/NYSE_Opening_and_Closing_Auctions_Fact_Sheet.pdf, https://www.nasdaqtrader.com/content/ProductsServices/Trading/Crosses/openclose_faqs.pdf