-well-filtered spaces, revisited

We prove that a T₀ topological space is -well-filtered if and only if it does not admit either the natural numbers with the cofinite topology or with the Scott topology as its closed subsets in the strong topology. Based on this, we offer a refined topological characterization for the -well-filterification of T₀-spaces and solve a problem posed by Xiaoquan Xu. In the setting of second countable spaces, we also characterise sobriety by convergences of certain ⁰₂-Cauchy subsets of the spaces.