Back in the glory days, as far as I remember of course, on SAA the Captain was the Captain, wherever he was. The licence system broke down into......
ATPL. No weight restriction.
SCPL. Weight restriction to be PIC.
CPL. Weight restriction to be CPL.
So a SFO would typically have an SCPL, a boy pilot a CPL and the Captain an ATPL.
Boy pilots were used to carry crew bags and were jolly handy to take messages back to pretty passengers and suchlike sundry items.