It means they're probably not selling F on that flight. They similarly don't sell F on several dates when the 744 operates this route (also, see HKG-DPS). I guess it's just not a route CX wants to sell F on (its own metal), presumably because there will likely be equipment substitutions to 2-class aircraft.

They may or may not assign seats in the F cabin, usually only to top-tier MPC members or others connecting on an F ticket, and usually only at the airport.