Halliday & Matthiessen (2014: 157):
… unlike modal Adjuncts, which are interpersonal in function, conjunctive Adjuncts are textual — they set up a contextualising relationship with some other (typically preceding) portion of text. The semantic basis of this contextualising function is that of the logical–semantic relationships of expansion. But the conjunctive Adjuncts construct these relationships by cohesion — that is, without creating a structural link in the grammar between the two parts.