Purely curiosity...

After a recent bump a CO agent at IAH printed out our itinerary on the dot-matrix printer. It included a column which showed "HK1" for each segment. This in addition to the column which usually contains OK and REQ????

2 Years ago MissBumpMe! and I ended up being bumped because a DL agent said "We have to get the HKs on board."

What are they talking about HK? HK1?