The Authors' Definition Of A Response

Martin & Rose (2007: 232-3):

We can thus define a response as a move which:

(1) takes as given the experiential content of its initiating pair part

and

(2) accepts the general terms of its argument established by its Subject-Finite structure (i.e. its polarity/modality/temporality). …

But a response does not allow for changes to the nub of the argument (its Subject),  or to the content of what is being argued about in the rest of the clause. By definition, any move making changes of this kind would not be considered a response but a new initiating move.

Blogger Comments:

[1] To be clear, speech function is concerned with interpersonal meaning, not with experiential meaning. In the case of propositions, what is accepted or rejected in a response is the validity of what is predicated of the Subject (Halliday & Matthiessen 2014: 148, 151-2).

[2] To be clear, a responding move may accept or reject the validity of a proposition or proposal. 

[3] Halliday & Matthiessen (2014: 148):

From above, [the Subject] is that which carries the modal responsibility: that is, responsibility for the validity of what is being predicated (stated, questioned, commanded or offered) in the clause. … The notion of validity relates to the arguing of the case, if it is a proposition, or to the putting into effect if it is a proposal. The Subject is that element in which the particular kind of validity (according to the mood) is being invested.