Halliday (2008: 49):
Interpersonally, the same two perspectives come into play. Some interpersonal meanings are highly generalised, like the enactment of dialogic rôles (speech function); Hasan (1992) and Hasan & Cloran (1990) present semantic networks for interpersonal systems of speech function (questions and commands) showing their realisations in the grammar.
With options in the way something is evaluated (“I approve / I disapprove”) or contended (“I agree / I disagree”), the borderline between grammar and lexis is shaded over; systems of appraisal, as described by Martin & White (2005), represent more delicate (more highly differentiated options within the general region of evaluation.
There are then mixed systems where the two perspectives intersect; for example systems of modality, in which the various degrees of probability and usuality have multiple realisations, including those where items which elsewhere function lexically are organised into systemic sets — wordings like certainly, perhaps, I think, I’m convinced and many others.