Halliday & Matthiessen (1999: 559):
As we saw, the grammar distinguishes a number of types of process, material, mental, verbal and relational; the distinctions are made by a cluster of syntactic variables — the valency of associated participant roles, the class of entity that takes on each role, the potential for combining with other figures, the associated tense systems and the like. But since these variables "draw the line" at different places, there are areas of overlap, with mixed categories that share some characteristics with one group and some with another.
We gave the example of behavioural processes; these are a mixed category, formed by the overlap of the material, on the one side, and the mental or verbal on the other. Behaving is construed as a type of figure that (like the mental) typically has a conscious participant as the central role, and does not extend beyond this to a second participant; but, on the other hand, it does not project, and it has a time frame like that of the material. Thus behavioural processes lie squarely athwart a fuzzy borderline.