Halliday & Matthiessen (2014: 143n):
The combination of Mood plus Residue embody the proposition or proposal of the clause (with the Mood element as the key to the distinction between the two); but, as we shall see below, there are certain interpersonal elements of the clause that do not belong to either the Mood element or the Residue: the Vocative, and comment and conjunctive Adjuncts. These relate to, but are not part of the proposition/proposal enacted by the clause.