contractURI function definition

I'm trying to understand where the standard for contractURI is defined. I was reading through some smart contracts and noticed it implemented. Upon searching and being directed here: https://docs.opensea.io/docs/contract-level-metadata I realize that OpenSea expects this function to define royalty for the ERC-721 contract.
Now my question is where this interface is defined, couldn't find it anywhere in the ERC-721 definition nor in the ERC-721 Metadata definition.