Back in the old days, newbies join as a 2-stripes F/O. Then you become a SF/O with two and half stripes. All ULH flights were on the classic and there were 5 crew, 2 capts., 1 F/O, and 2 SF/Es.
The present structure at CX is way too complicated. It makes no difference whether I fly with 2 stripers or 3 stripers. I got to admit the term JFO sounds a lot nicer than Acting F/O.