My understanding (as a non CX dude) is that its a seniority based operation, and the pax and freighter fleet newbies are employed on a common seniority list, even though the freighter is a different contract/operation.
Eligibility for promotion eventuates when your number comes up, regardless of fleet.
I think also the freighter contract provides a little less cash in the pocket, however you do get pole time.
Anybody in the know care to correct me?