Halliday & Matthiessen (1999: 511):
We have stressed all along that a language is a system for creating meaning; and that its meaning potential has evolved around three motifs — what we refer to as the “metafunctions” of ideational, interpersonal and textual, with the ideational in turn comprising an experiential component and a logical component. These are the multiple aspects of the content plane — the grammar (in its usual sense of lexicogrammar) and the semantics. Since the powerhouse of language lies in the grammar, we shall refer to them here as aspects of the grammar; but it is important to insist that they could not be “in” the one without also being “in” the other. It makes no sense to ask whether the metafunctions are grammatical or semantic; the only possible answer would be “yes”.
