According to Sonia’s interpretation, Peacocke is committed with objects (possibilia) that are contingently non concrete (they are not concrete, but they could have been concrete.) So, a possibile exists in a possible world just in case that possible world involves certain non concrete object. That possible world would involve a singular Russellian proposition containing that object.
According to my interpretation (which corresponds to the theory that anyway I would like to favor), the principle-based account wasn’t committed with such mysterious entities. That a possibile exists in a possible world simply means that the existential quantification of its individuating condition holds in that world. So, a possibile exists in a possible world just in case that world involves the existential proposition that there’s something satisfying certain property (certain individuation-condition). But this world won’t involve the singular Russellian proposition that certain (non actual) object satisfies that property.
Now I’m not so confident about this interpretation, because in pp. 501-2 he seems to hold that he is not trying to resolve this issue. In any case, it would be interesting if someone could provide more textual evidence and/or an opinion about this issue.