My senior once told me polyclinic is like national service, that everyone has to go through.

Mopex with mbbs= recruit, do all the ground work (i.e see all the full of nonsense pts)
Gdfm= corporal, i.e for those who cannot make it to command school

Mmed = sergeant (does sai Kang arrowed to them, also have to guide recruits)
Fellowship= officer (do all the higher level planning office work, arrow things to their mmed sergeant and recruits)

Residency training = sispec or Specialist cadet
Fellowship training = OCS

Once your time is up, can ord and leave and go join outside world...