Martin (1992: 412):
Unlike ideational metaphors, interpersonal metaphors are not so much concerned with packaging information as Theme or New as with what Halliday has described as modal responsibility (1985: 76-8), 1984/1988: 39-45) — they arrange the Mood functions which are appropriate for particular interacts. Because of its ineffability this interpersonal texturing needs to be explored in context.
 Like ideational metaphors, interpersonal metaphors are also concerned with providing alternative groupings of quanta of information, as when modality and speech function are incongruently realised through interpersonal projection (I think, I urge); see Halliday & Matthiessen (2004: 626-7).
 To be clear, modal responsibility is carried by the Subject. As part of identifying the Subject from a trinocular perspective, Halliday & Matthiessen (2004: 119) write:
From above, it 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 Mood element, on the other hand, functions as the nub of the proposition. Halliday & Matthiessen (2004: 120):
Hence the Mood element has a clearly defined semantic function: it carries the burden of the clause as an interactive event. So it remains constant, as the nub of the proposition, unless some positive step is taken to change it… This is an uncomprehending reference to Halliday's 1984 paper On The Ineffability Of Grammatical Categories. The word ineffable can also be used attitudinally:
“What ineffable twaddle!” I cried, slapping the magazine down on the table; “I never read such rubbish in my life.”
— Dr. John H. Watson to Sherlock Holmes in Arthur Conan Doyle's A Study In Scarlet