Taking a stab in the dark but it sounds like something along the lines of all reservations having to be done on request to CX. Meaning that if booked by a TA or another carrier only a message requesting a seat can be sent to CX as opposed to automatic confirmation if seats are available.

That flight number looks suspiciously like a combined freight/pax service. (For another example look up HKG-KIX). They usually don't operate these flights every day and they depart at odd times (mostly overnight). Because these flights are predominantly for freight the number of pax carried is tightly restricted for weight reasons.