HK are passengers who purchased a ticket but did not select a specific seat. HKI are passengers who did not get on their ticketed flight and are standing by for the next or a different flight.

The current process as it plays out for many of the "connection carrier" flights shows over 1/2 of the seats as preferred seating and it costs the passenger extra if they choose one. If they don't choose a specific seat, they will be assigned one at the gate (watch the screen) and if it turns out to be in an exit row, they don't have to pay the extra money. If the flight is oversold, these HKs don't get on and then roll over to another flight and become HKIs. (HKI's can also be passengers who missed their connection etc.)

Check out travel net, find a flight far enough into the future where few or no seats have been sold. Then go on an expedia type site and find the flight. Look at the seating chart. From the flights I've looked at, it appears that everything on a 50-76 seat RJ, from the over wing exit seats forward, is the "preferred seat" catagory and it costs an additional fee. Also check a flight for this weekend and compare the difference. You can play around a lot on those sites without having to furnish your CC and it is very educational for non-reving.