Halliday & Matthiessen (2014: 361):
(1) Transitivity structures express representational meaning: what the clause is about, which is typically some process, with associated participants and circumstances;(2) Mood structures express interactional meaning: what the clause is doing, as a verbal exchange between speaker/writer and audience;(3) Theme structures express the organisation of the message: how the clause relates to the surrounding discourse, and to the context of situation in which it is being produced.These three sets of options together determine the structural shape of the clause.