Let $X$ be a stack of $n$-groupoids on the site of affine schemes over a fixed base, with the etale topology. If $n=1$ then for $X$ to be Deligne-Mumford, aside from having an etale atlas from an algebraic space, one must also impose certain separability conditions on its diagonal. In DGA-V, Lurie remarks that what he defines as higher Deligne-Mumford stacks lack  separation axioms, but that they may be added in by hand later. My question is, what separation axioms should be added? Here, I do not mean "separable" or "quasicompact". What I mean to ask is on which morphisms do I put the appropriate separability conditions? It seems to be that simply imposing them on the diagonal may be too naive, but perhaps I am wrong, hence this question.