Halliday & Matthiessen (2014: 190, 192):
With this type, the speaker is commenting either on the proposition as a whole or on the part played by the Subject. In the first case, the comment may be either asseverative (‘it is so’; typically tone 1) or qualificative (‘this is what I think about it’; typically tone 4). These items cannot function as circumstantial Adjuncts: it makes no sense to say it happened evidently. …
In the second case the Subject’s rôle is being evaluated for its wisdom or morality, or typicality; such expressions can occur circumstantially (contrast wisely, he didn’t act, comment Adjunct, with he didn’t act wisely, circumstance of Manner); …
Such subject-oriented comments may also be expressed as predications, through verbal group complexes serving as Predicator …