Halliday & Matthiessen (1999: 381):
The grammar unifies the different metafunctional contributions; for example, a figure, a move and a message are unified in their realisation as a clause. It achieves this unification by realising combinations of ideational, interpersonal and textual features in the same wording. For example, the wording well unfortunately they must have missed the train is a realisational unification of a figure (of doing & happening), a move (of giving information, assessed as certain and undesirable) and a message.