Halliday & Matthiessen (1999: 4-5):
Language, therefore, is a resource organised into three strata differentiated according to order of abstraction. These strata are related by means of realisation. Semantics, or the system of meaning, is realised by lexicogrammar, or the system of wording (that is, grammatical structures and lexical items); and lexicogrammar is realised by phonology, or the system of sounding. [This is the traditional formulation; more properly: semantics is realised by the realisation of lexicogrammar in phonology.]
For instance, a sequence of figures (a sequence of configurations of processes with participants and attendant circumstances) at the level of semantics is realised by a complex of clauses at the level of grammar; this, in turn, has its own realisation in the phonology — for example, a particular complex of clauses might be realised by a particular sequence of tones (pitch contours).
Between lexicogrammar and phonology runs the line of symbolic arbitrariness: prototypically the relation between these two levels is conventional, whereas that between semantics and lexicogrammar is prototypically natural. What this means is that experience is construed twice in the content plane, once semantically and once lexicogrammatically. The ideational meaning base that we are concerned with in this book is a construct that is 'located' within the semantic system — that is, at the highest level — and realised in the lexicogrammar.