Halliday & Matthiessen (2014: 156):
These are the modal Adjuncts [in Mood or Comment] and the conjunctive Adjuncts [not in mood structure]. … The distinction among these different kinds of Adjuncts is a metafunctional one. … Modal and conjunctive Adjuncts are, respectively, interpersonal and textual in metafunction; hence they occur at different locations within the clause.