The surrogate model is updated in each iteration of BO, after the new observation (x n+1 , y n+1 = f (x n+1 )) is gathered, to (i) improve the approximation of f (x) and (ii) account for the reduced uncertainty in the region of the feature space surrounding the newly evaluated NPM x n+1 . Consequently, let us denote the surrogate model after iteration n of BO as fn : x → (ŷ , σ).