I expounded my theory last month (someone should make this topic
a sticky) but, again, to quote from a 1992 "What Parents Should
Know" NS primer copy I own:
"II. Vocational Training
For those who are not selected for any of the Junior Commander
Courses after their BMT, they will be trained for a specific
vocation.
The SAF matches servicemen with vocations according to its
organisational needs. The following points are considered in the
designating of vocations:
a. Educational qualifications.
b. Working experience.
c. Physical characteristics, eg. height & weight.
d. Physical fitness.
e. The SAF's organisational needs.
f. Unit Commander's recommendation.
Each vocation also has a job description stating the type of men
required the job. If there are no vacancies in the vocation a
serviceman has been selected for, he will be matched with the next
suitable vocation."
I suggested further that perhaps each of these categorical grades
gets mathemetically categorized into a computer so that vocations
could be impersonally assigned in a jiffy.
As such your civilian credentials & the pre-enlistment checkup
(medical classification, aptitude test) both do have some
influence, as do your DIs' feedback in BMT. But note that the
general PES status & the conditional FFI criteria (alluded to
in the last paragraph of the quote) are separate designations. You
could be PES A-fit but won't be able to get posted to, say, for the
GDS Conversion Course if you don't pass their FFI.