Those who are holding Service Spec vocation are the one who have pass out from their Spec course. Since their Route of Advancement never involve in command school, they can be promoted to 3SG if they pass their IPPT or exempted.
CFC is never a MUST to achieve rank between CPL & 3SG rank. CFC is for those 'top percentage' CPL holding man vocation & one of the requirement is to get IPPT Gold.
I strongly suspected this is propose by some NSF who is holding man vocation working in higher HQ that can easily get approval by higher commander but his Pes does not allow him to go IPPT & the competition is a group level.
This is because who will set a ridiculous requirement such as IPPT Gold unless he can achieve it or exempted from IPPT. Mostly those who can get IPPT Gold will be at combat fit related unit which the competition will be at Platoon or Company level which I strongly doubt they will set a ridiculous requirement to be the top percentage of CPL rank.
I thought now SAF no longer offer 3SG rank to those whom never went to command school. Thats why they came out with the new rank CFC to bridge the gap likely for those in the admin service vocation whom deserve to be promoted from a CPL rank after attending a further course.