There are many interface contract.
I want to know why IERC721.sol is needed?
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Hi @Shinsaku,
ERC721 defines the different interfaces: https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
The OpenZeppelin Contracts API documentation covers these interfaces:
https://docs.openzeppelin.com/contracts/2.x/api/token/erc721
As per the OpenZeppelin documentation, only IERC721 is required in a contract to be ERC721 compliant.
Feel free to ask all the questions that you need.
Hi @Shinsaku,
Did the above reply answer your question?
If it did, can you mark it as the solution please 
Otherwise, add follow up questions if you need.