Halliday & Matthiessen (2014: 361-2):
At the same time, in interpreting group structure we have to split the ideational metafunction into two modes of construing experience: experiential and logical. So far what we have been describing under the ideational heading has been meaning as organisation of experience; but there is also a logical aspect to it – language as the construal of certain very general logical relations – and it is this we have to introduce now. The logical component defines complex units, e.g. the clause complex … and group and phrase complexes … . It comes in at this point because a group is in some respects equivalent to a word complex — that is, a combination of words built up on the basis of a particular logical relation. This is why it is called a group (= ‘group of words’).