Definition 98.18.1. Let S be a scheme. Let \mathcal{X} be a category fibred in groupoids over (\mathit{Sch}/S)_{fppf}. We say \mathcal{X} satisfies condition (RS*) if given a fibre product diagram
\xymatrix{ B' \ar[r] & B \\ A' = A \times _ B B' \ar[u] \ar[r] & A \ar[u] }
of S-algebras, with B' \to B surjective with square zero kernel, the functor of fibre categories
\mathcal{X}_{\mathop{\mathrm{Spec}}(A')} \longrightarrow \mathcal{X}_{\mathop{\mathrm{Spec}}(A)} \times _{\mathcal{X}_{\mathop{\mathrm{Spec}}(B)}} \mathcal{X}_{\mathop{\mathrm{Spec}}(B')}
is an equivalence of categories.
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