Halliday & Matthiessen (1999: 135-6, 137):
As with all systems in language, any given instance will be more or less prototypical; and there may be subtypes lying immediately at the borderline of the primary types. The grammar construes the non-discreteness of our experience by creating borderline cases and blends. One such area is that of behavioural processes (Halliday 1985: 128-9): “processes of physiological and psychological behaviour, like breathing, dreaming, smiling, coughing”. […] Such borderline cases, in which the pattern of reactances does not conform exactly to that of a major type, are typical of grammatical systems in general.