There are Seven RIAF airmen commemorated on CWGC with the rank of Improver 1st Class or Improver 2nd Class

I could not find much data on this rank - what do they do?

I note that their 4 digit service number is different from the RIAF airmen service numbers which is five digits.

this seems India specific and only related to the RIAF. I suspect some type of non combatant rank?