What does Quantum Phase Estimation estimate?

The amplitude α where U|ψ⟩ = α|0⟩ + β|1⟩

The eigenvector |ψ⟩ of the unitary U itself

The global phase e^(iφ) of the whole register

The eigenphase θ where U|ψ⟩ = e^(2πiθ)|ψ⟩