Martin (1992: 464):
Seen in these terms negotiation involves resolving an exchange by replaying its Mood function (possibly fine tuning through tone), adjusting its POLARITY, MODALITY or TENSE, or substituting its Subject or part of its Residue; alternatively the initiating proposal or proposition has to be replaced and negotiation started all over again. The interlocutor initiating the exchange tries to facilitate this by centring meanings at risk in the Mood; it is this facilitation process that explains the pattern of Subject choice in dialogue — the Subject, in other words, is what is at stake.
 To be clear, it is the Mood block, the Subject and Finite, that makes a proposition arguable. Halliday & Matthiessen (2014: 212):
The ‘nub of the argument’ is realised by the Subject of clause and the Finite fixes it … in relation to the now of speaking.
 In considering the clause in its interpersonal semantic guise, as a move in an exchange, what's "at stake" or "at risk" — that is, what is arguable — is not the Subject, but the validity of the proposition or proposal. The Subject is the element on which this modal responsibility rests; the validity of a proposition is stated or questioned with respect to the Subject.