Example implementation: https://rinkeby.etherscan.io/address/0x998Cf6fDd7FF1EB258c288762284142D28D0ee39#code
EIP1271 allows a contract to delegate authorized signers. Usually EIP1271 expects an authorized delegate to submit a signature via the contract, and uses typical ECrecover to see if the signature is valid.
EIP1271 returns valid signature: true**
if the signature is valid & the signer is authorized by the contract
** not actually true
, but instead a magic value to guard against mistaken 'true' bit
The Rarible contract checks if the signature is valid with the following code
require(
ERC1271(order.maker).isValidSignature(_hashTypedDataV4(hash), signature) == MAGICVALUE,
"contract order signature verification error"
);