Halliday & Matthiessen (1999: 417):
In the logico-philosophical orientation, the basic unit tends to be determined “from below”, from grammar: since sentences are seen as propositions, the basic unit of semantics is the proposition (as in propositional calculus). In contrast, in the rhetorical-ethnographic orientation, the basic unit tends to be determined “from above”, from context: since language is seen as functioning in context, the basic unit of semantic[s] is the text (see Halliday & Hasan 1976; Halliday 1978). So in the logico-philosophical orientation, semantics means in the first instance propositional semantics, whereas in the other orientation it means text or discourse semantics.