In some versions of semantics for conditional logics employing possible worlds, a binary function $f$ that selects possible worlds corresponding to each pair $〈w,\phi 〉$, where $w$ is a possible world and $\phi$ a formula. The intended interpretation of the value of $f\left(w,\phi \right)$ is given in terms of similarity as the set of possible worlds at which $\phi$ holds that are most similar to $w$. Whether $f\left(w,\phi \right)$ may include more than one possible world will be decided by the semantic conditions that one imposes on the model. Generalizations of the selection function include the set selection function, analogous to neighbourhood semantics for modal logic insofar as the set selection function maps each pair $〈w,\phi 〉$ to a neighbourhood, i.e., a set of sets of possible worlds.