 This means that constructing the pullback of a quasicoherent sheaf under a morphism of stacks requires some extra effort.It is also possible to use finer topologies.Most reasonable "sufficiently large" Grothendieck topologies seem to lead to equivalent categories of quasi-coherent sheaves, but the larger a topology is the harder it is to handle, so one generally prefers to use smaller topologies as long as they have enough open sets.