I find the biggest source of confusion in non revenue travel is people ignoring the actual bookings and going to the seat availability page. If a flight with 160 seats is booked at 160 but shows 30 seats unassigned it's not a good bet. In the hours before the flight there will be 30HK's show up on the airport standby list and the flight will be full. The difference between booked seats and assigned seats becomes the dreaded HK's. There are a lot more booked seats without seat assignments in the last few years because the new cheap economy tickets do not allow advance seat selection. They are always HK's the day of the flight.