When anybody joined the RCAF as an Airman, they received a Regimental Number starting with an 'R'. If they were commissioned during their service life, or if they were commissioned on entry, they received a new Regimental Number starting with a 'J'.
Can someone tells me why some RCAF personel has a R/ or a J/ before their personel numbers ?
As John correctly stated, R numbers were for enlisted personnel and J numbers were assigned to commissioned airmen. Another one of note is C prefix which was assigned to commissioned members that were in the service prior to the war. I am not certain at which point in time that the use of C as prefix stopped.