My TA once showed me the various codes that appeared on the GDS screen during the booking process. "HKx" was the final step, standing (IIRC) for Hold Keep x seats (eg HK1, HK2). That was reached when the airline confirmed the seat(s) as reserved in that particular PNR and gave an airline locator for them.
That ought to translate into "OK" on the ticket. I've never seen "HK" appear on an actual ticket - only "OK" for a confirmed booking - although "HK" has been on various computer printouts I've been given.