 The Mares-Goldblatt semantics is called an “admissible set” semantics. The propositions are the admissible sets. In some models at least, not every set of worlds counts as a proposition. One rationale for that comes from reflection on how humans relate circumstances together as similar. Not every set of situations are such that we would see a similarity that the members of the set have to one another and to no other situations outside the set. It seems reasonable to think of a proposition as a set of worlds that could act as a content for some person. (Perhaps if we were to construct a language to talk about what people could think in other words, we could index sets of propositions to worlds. But that is a topic to be left for some other time.) 