Let $f$ be a rational piecewise affine function on $N_{\mathbb{R}}$, and let $\Pi$ and $\{(m_{\Lambda},l_{\Lambda})\}_{\Lambda\in\Pi}$ be as in Proposition 3.89. The family $\{(m_{\Lambda},l_{\Lambda})\}_{\Lambda\in\Pi}$ is called a set of defining vectors of $f$.