Thread: What does a - mean in the loads?

It means "don't even bother"

In all seriousness, airlines overbook because they know that people will no-show or change their flight. SFO-HKG is definitely one of those routes that can be quite overbooked because of all the business travelers that fly on that route (and change plans). When availability is negative, that means that there are more people booked than they have seats.

BUT you will also see that there are 3 seats held in F, which is accounted for when giving the number of available seats. That's probably there to save room for oversells in C (more profitable to take an extra pax in C and upgrade them than to sell them in Y or lose them altogether). One of those 3 seats is probably for crew rest, though.