Halliday & Matthiessen (2004: 248-50):
These are processes of (typically human) physiological and psychological behaviour, like breathing, coughing, smiling, dreaming and staring. They are the least distinct of all the six process types because they have no clearly defined characteristics of their own; rather they are partly like the material and partly like the mental.
Halliday & Matthiessen (2004: 255):
… ‘behavioural’ process clauses are not so much a distinct type of process, but rather a cluster of small subtypes blending the material and the mental into a continuum …