Circulating Supply is basically the number of tokens that are available to the public and moving from one hand to another on crypto-market, but you get to understand it better if you look at it from code level.
It seems to me that at the smart contract level, Circulating Supply would have different definitions in different context of contract logic. Therefore I will categorise contract logic based on whether there is an initial total supply or not.
• Initial Total Supply
For tokens with an initial total supply, the initial supply is assigned to an address or a number of addresses which are in control of the contract creator.
If the creator of the token decides to sell or airdrop some of the tokens, those tokens would be assigned to new addresses. Hence, all the number of tokens that are no longer owned by the contract creator but are assigned to new addresses is the Circulating Supply.
In other words, the Circulating Supply is the number of tokens that are not assigned to addresses owned or in control by the contract creator but are assigned and are in control by other people.
• No initial Total Supply
For tokens with no initial total supply, the Circulating Supply is simply the number of tokens that are minted and assigned to addresses.
One should take notice of one thing that runs through the two definitions presented in the two context of contract logic: the Circulating Supply is the tokens not in the hands of the contract creator.
