Halliday & Matthiessen (2014: 49):
Each system has its point of origin at a particular rank: clause, phrase, group and their associated complexes. … the clause is the primary channel of grammatical energy … . Systems at every rank are located in their metafunctional context; this means, therefore, that every system has its address in some cell of a metafunction-rank matrix … . For example, the system of MOOD is an interpersonal system of the clause; so it is located in the ‘clause’ row, ‘interpersonal’ column in the matrix.