= Solution
A <Weil divisor> on $X$ is a finite formal sum
$$
D=\sum_Zn_Z[Z],\qquad n_Z\in\mathbb Z,
$$
over integral codimension-one closed subschemes $Z$. Since $X$ is <regular in codimension one>, the local ring at the generic point of each $Z$ is a <discrete valuation ring>. A nonzero <rational function> $f\in k(X)^\times$ therefore defines the principal divisor $\operatorname{div}(f)=\sum_Zv_Z(f)[Z]$. The <divisor class group> is
$$
\operatorname{Cl}(X)=\operatorname{Div}(X)/\{\operatorname{div}(f):f\in k(X)^\times\}.
$$
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