Looking for clarification on the distinction between subjuncts and disjuncts, in certain uses

Question

In the grammar book I am using, subjuncts are defined as playing a subordinate role in a sentence. One of these uses, as given in the book, is to express a point of view:

"Morally, he should resign."

On the following page the book gives this example for a disjunct:

"Frankly, John should never have done it."

Style disjuncts express the conditions under which the listener/reader is to interpret the sentence. My question is borne out of a confusion: how do I distinguish between these two examples? They seem very similar to me.

Answer 1

[1] Morally, he should resign.

[2] Frankly, John should never have done it.

In [1] the adverb “morally” is a domain adjunct, the kind that restricts the domain to which the rest of the clause applies. The property of being obliged to resign is ascribed to the referent of "him" only with respect to some moral issue; in other respects he may be fine at his official duties.

By contrast, in [2] “frankly” is a speech act-related adjunct. The adverb “frankly” describes my speech act; the sentence can be glossed approximately as “I tell you frankly that John should never have done it”. The adverb doesn’t mediate the way in which the proposition relates to truth. The proposition that John should never have done it is presented as a fact. Even if I’m not actually speaking frankly, the residue would not be false.