The Ethereum Community has adopted several standards to help keep projects interoperable across implementations and keep smart contracts composable. ERC721 is the standard that is used to define Non Fungible Tokens distributed from the contract. This could be due to its age, rarity, or something else like a visual representation. All NFT’s have a uint variable “tokenId” that makes that token unique(Non Fungible). Many times, this “tokenId” is used along with a converter to generate unique inputs and outputs to the application. This is what “Crypto Kitties” did.