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Given a category $\mathcal{C}$, the category of elements functor sets up an equivalence of categories $$ \mathsf{DFib}(\mathcal{C}) \cong \mathsf{PSh}(\mathcal{C}), $$ whereas the Grothendieck construction sets up a $2$-equivalence $$ \mathsf{CartFib}(\mathcal{C}) \cong \mathsf{PseudoPSh}(\mathcal{C}). $$

Question: If one puts a Grothendieck topology $\mathcal{T}$ on $\mathcal{C}$ and replaces the right sides of the above ($2$-)equivalences with $\mathsf{Shv}_{\mathcal{T}}(\mathcal{C})$ and $\mathsf{Stacks}_{\mathcal{T}}(\mathcal{C})$, what should the corresponding categories on the left be?

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    $\begingroup$ Not sure if this is the answer you're looking for, but the essential image of the Grothendieck construction in fibered categories can be described as those where "morphisms glue and descent data are effective", compare stacks.math.columbia.edu/tag/0268. $\endgroup$ Commented Mar 18, 2021 at 23:05
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    $\begingroup$ "The discrete fibrations that are sheaves". (-: The sheaf condition can be expressed equally well on either side of the equivalence. $\endgroup$ Commented Mar 19, 2021 at 0:59
  • $\begingroup$ @BertramArnold Thanks! I was looking exactly for a characterisation of the essential image :) $\endgroup$
    – Emily
    Commented Mar 19, 2021 at 3:31
  • $\begingroup$ @MikeShulman Thanks; this is very nice! How is the sheaf condition defined for discrete fibrations? $\endgroup$
    – Emily
    Commented Mar 19, 2021 at 3:32
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    $\begingroup$ The same way it's defined for contravariant functors to Set. $\endgroup$ Commented Mar 19, 2021 at 15:06

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The essential image of the Grothendieck construction from (weak) functors $C\to\operatorname{Cat}$ which are sheaves with respect to a Grothendieck topology $\mathcal T$ is described in Section 8.4 of the Stacks project, with the full subcategories of sheaves of groupoids and sets (more precisely, "setoids", i.e. groupoids such that all morphism sets have at most one element) described in the following sections Section 8.5 and Section 8.6, respectively.

To be precise, these sections define stacks, but you can probably compare this definition with your favourite one using the description of the Grothendieck construction in Section 4.36.

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