The true label of an example is y(x) = 1 if the example is positive and y(x) = −1 if the example is negative.
x ∈ P ⇒ s(x) = y(x) = 1 and x ∈ U ⇒ s(x) = −1 i.e., the true label y(x) of an unlabeled example is unknown.