HK is a term used to determine the status of the booking, I always refered to HK as Hold Konfirmed (i.e. ticketed and booked) there are other status to indicate Stand-by, and non-rev/employee reservations but I dont know what they are. This is also followed by the number of people (for example HK1 referes to one person, HK4 refers to 4 people). There is no other higher or perfered status then HK that I know of.