Most venues let you mark an order as auction-only so it sits out of the continuous session entirely. market-on-open, limit-on-open, market-on-close and limit-on-close are all forms of it, as are halt-auction-only orders.
The point is benchmark discipline. A fund promising clients the official close does not want half its size printed at 3:58 pm at a different price.
Example: you enter an auction-only buy at 25.00 at noon. The stock trades down to 24.50 at 2 pm and your order does nothing, because it is not in the continuous book. At the close the cross prints 24.80 and you fill there. Some venues also offer imbalance-only orders that may only offset a published imbalance.