Halliday & Matthiessen (2014: 193):
The networks of mood and comment Adjuncts are drawn up in the perspective ‘from the same level’: they encompass just those items that function as interpersonal Adjunct. Thus they do not include expressions from the same semantic domain which do not function as Adjuncts: typically non-finite clauses, for example to be honest, to tell you the truth, come to think of it. Such expressions would be included in a network drawn up in the perspective ‘from above’.