The sign function converts a linear score into a binary label, mapping positive and negative values to +1 and −1. In a perceptron, h(x; θ) applies it to θ·x + θ₀. section 5.2
Linear decision boundary
The equation θ·x + θ₀ = 0 defines the decision boundary. Points on the normal-vector side receive +1; points on the opposite side receive −1. section 5.2

