What value should be used for "ownershipProof"

What should ownershipProof value be when using registerAddress extrinsic? It’s not mentioned in the whitepaper. In fact, the whitepaper section about registerAddress seems to be for the CTC 1.0 implementation.

It is a signature signed by the key that’s used to generate the address, thereby showing proof that the address is owned by the submitter.

Yeah, but what should be signed?