From the Cardano-developer website:   "Since asset names are not unique and can be easily duplicated, Cardano NFTs need to be identified by the policyID. This ID is unique and attached permanently to the asset. The policy ID stems from a policy script that defines characteristics such as who can mint tokens and when those actions can be made.   Many NFT projects make the policyID under which the NFTs were minted publicly available, so anyone can differentiate fraudulent/duplicate NFTs from the original tokens.   Some services even offer to register your policyID to detect tokens that feature the same attributes as your token but were minted under a different policy."