During the Pre-opening Session and the Closing Auction Session, the trading system of the Exchange accepts only At-auction Order and At-auction Limit Order.
An At-auction Order is an order with no specified price and is entered into the trading system for execution at the final Indicative Equilibrium Price (IEP). It enjoys a higher order matching priority than an at-auction limit order and will be matched in time priority at the final IEP. Any outstanding at-auction orders after the end of the Pre-opening Session will be cancelled before the commencement of the Continuous Trading Session.
An At-auction Limit Order is an order with a specified price. An at-auction limit order with a specified price at or more competitive than the final IEP (in case of buying, the specified price is equal to or higher than the final IEP, or in case of selling, the specified price is equal to or lower than the final IEP) may be matched at the final IEP subject to availability of eligible matching order on the opposite side. An at-auction limit order will be matched in price and time priority at the final IEP. No at-auction limit order will be matched at a price worse than the final IEP. Any outstanding at-auction limit orders at the end of the Pre-opening Session will be carried forward to the Continuous Trading Session and treated as limit orders provided that the specified price of that at-auction limit order does not deviate nine times or more from the nominal price or is one ninth or less of that price. Such orders will be put in the price queue of the input price.
During the Continuous Trading Session, the trading system of the Exchange accepts only Limit Order, Enhanced Limit Order and Special Limit Order.
A Limit Order will allow matching only at the specified price. The sell order input price cannot be made at a price below the best bid price, if available, whereas the buy order input price cannot be made at a price above the best ask price, if available. Any outstanding limit order will join the price queue of the input price.
An Enhanced Limit Order is similar to a Limit Order except that it will allow matching of up to 10 price queues (i.e. the best price queue and up to the 10th queue at nine spreads away) at one time provided that the traded price is not worse than the input price. The sell order input price can be made at a price of nine spreads below the current bid price whereas the buy order input price can be made at a price of nine spreads above the current ask price. Any outstanding enhanced limit order will be treated as a limit order and put in the price queue of the input price.
A Special Limit Order will allow matching of up to 10 price queues (i.e. the best price queue up to the 10th queue at nine spreads away) at one time provided that the traded price is not worse than the input price. A special limit order has no restriction on the input price as long as the order input price is at or below the best bid price for a sell order or at or above the best ask price for a buy order. Any outstanding special limit order will be cancelled.
Investors may refer to here for details.