Premarket is the pre-opening session of Hong Kong stocks market.
In the pre-opening session, orders are accumulated over a certain period of time and matched at a pre-defined order matching period. Orders are matched in order type, price and time priority (at-auction orders carry a higher matching priority), at the final Indicative Equilibrium Price ("IEP"). 

During the Pre-opening Session, the system accepts at-auction and at-auction limit orders only. Any outstanding at-auction orders after the end of the Pre-opening Session will be cancelled before the commencement of the Continuous Trading Session.