A function $f$ on $X$ is a model function if there exists a model $\mathcal{X}$ and a $\mathbf{Q}$-divisor $D\in\Div_{0}(\mathcal{X})_{\mathbf{Q}}$ such that $f=f_{D}$. We then call $\mathcal{X}$ a determination of $f$. We let $\mathcal{D}(X)=\mathcal{D}(X)_{\mathbf{Q}}$ be the space of model functions on $X$.