 If this was a typical exchange, Barb meant that she is not going to Paul’s party by saying that she has to work. She did not say that she is not going to Paul’s party, and the sentence she uttered does not mean that. Grice introduced the technical terms implicate and implicature for the case in which what the speaker said is distinct from what the speaker thereby meant or  implied.[1]  Thus Barb implicated that she is not going; that she is not going was her implicature.