The function selector are the first four bytes of the keccak256 hash of the function signature. What is the signature of the fallback function? It neither has a name nor any inputs.
If there is no real signature there have to be at least a custom function selector (some fixed 4 bytes reserved for the fallback function).
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There is no selector. The fallback function is called if the contract does not recognise the selector, or if no selector is provided.
