What is the function selector of the fallback function?

Question

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).

Answer 1

There is no selector. The fallback function is called if the contract does not recognise the selector, or if no selector is provided.