The restriction maps $N^{1}({{\mathscr{X}}}/S)\rightarrow N^{1}(X),\,[{\mathscr{L}}]\mapsto[{\mathscr{L}}|_{X}]$ induce a linear map $\{\phantom{a}\}:\mathcal{Z}^{1,1}(X)\longrightarrow N^{1}(X),\,\theta\mapsto\{\theta\}$. We call $\{\theta\}$ the de Rham class of $\theta$.