Understanding is a matter of making inferences to the best explanation. E.g., Echo in phone line means the call is long-distance.
Meaning is tied to the inferential process.
Difference depends on the way the effect is produced. But S does not mean this by virtue of her recognition of her own intentions. That would lead to a regress of intentions. We need a theory of mental content and this may leave room for an alternative extensionalist account. There can be no analysis of meaning which does not take into account what the speaker is trying to do with the words.