You mean this http://www.flyertalk.com/forum/catha...e-offered.html

Think your answer's in 4.3 of FAQ http://www.flyertalk.com/forum/15321330-post6.html

"there is a dedicated crew tending to the F section" I can't remember where, but I read this is actually a safety requirement.

So the following applies:

"Please note that the F cabin will only open for seating on two-class flights if Business Class passengers cannot be accommodated in the normal Business Class cabin."

i.e. you've got to have a somewhat full J cabin before you have a chance of getting into the F cabin. Otherwise CX can save on a couple of crews' duty pay on the flight.