Halliday & Matthiessen (2014: 190):
There is no very clear line between these and the Mood Adjuncts; for example, the ‘comment’ categories of prediction, presumption and desirability overlap semantically with the mood categories shown under modality. The difference is that comment Adjuncts are less closely tied to the grammar of mood; they are restricted to ‘indicative’ clauses (those functioning as propositions), and express the speaker’s attitude either to the proposition as a whole or to the particular speech function. In other words, the burden of the comment may be either ideational [propositional] or interpersonal [speech functional].
