Halliday & Matthiessen (1999: 417):
The approaches differ with respect to what they take as the basic unit of meaning.
In the logico-philosophical orientation, the basic unit tends to be determined "from below", from grammar: since sentences are seen as encoding 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 is the text (see Halliday & Hasan, 1976; Halliday, 1978a).So in the logico-philosophical orientation, semantics means in the first instance propositional semantics, whereas in the other orientation it means text semantics.