Halliday & Matthiessen (2004: 588):
This is the region where semantic units are realised by lexicogrammatical ones. … The clause is a multifunctional construct in the grammar, one that realises three different semantic units, one for each metafunction: textual — message, interpersonal — proposition or proposal, and experiential — figure. … The three semantic units deriving from the three metafunctions are all mapped onto the clause, which thus unifies the three metafunctional strands of meaning.