HK - Holding Confirmed (Reservation)
These folks are holding a reservation but do not have a seat assignment.
They are placed on the Airport Standby List with high priority and the gate agent
will assign a seat in their class of service or upgrade /downgrade if that is not
available. They are cleared ahead of all standbys.
An explanation of how they show up is when booking a reservation they may not
be able to select a seat if the flight is full and the emergency and bulkhead rows
are blocked out pending gate agent assignment. There are seats for these folks
somewhere on the flight - if not they are oversales and carry this HK status to
the next flight.