Circulating Supply is the most accurate approximation of the number of coins currently circulating in the market as well as in the general public's hands.
Total Supply is the total amount of coins that are existence at the moment (minus any coins that have been verifiably burned). Final Total Supply is the Max Supply minus all burned WINGs and WINGs to be burned, which is 5,000,000 now.
Max Supply is the most accurate approximation of the maximum amount of coins that will ever exist during the lifetime of the cryptocurrency. WING Max Supply originally is 10,000,000. In accordance with the WIP-08 voting results, WING's Final Total Supply has been lowered to 5,000,000.
This API is under development.