While digitally signing a document, the sender uses his private key, ${K_b}^-$ to computer ${K_b}^-(m)$.
It is unusual that sender is using his private key (which is generally used for decrypting a message), but sender's goal is not to scramble the contents of the document, but rather to sign a document in a manner that is verifiable and nonforgeable.
One concern with signing data by encryption is that encryption and decryption are conceptually expensive. A more efficient approach is to introduce hash functions into digital signature. Using hash function, Sender signs the hash of the message rather than message itself, thus sender calculates ${K_b}^-(H(m))$.
So the answer is $\left \{ {m, {K_b}^-(H(m))} \right \}$