Halliday & Matthiessen (1999: 217):
Obviously, choices made in all these parameters relate to the figure as a whole (at least; they may have implications for longer stretches of discourse). The major process types — the primary categories of sensing, saying, doing & happening, being & having — tend to have inherently different temporal characteristics, affecting all aspects of the way time is construed: in terms of what temporal categories are possible, what the relative probabilities are, and what the different choices mean (note for example the total lack of proportionality in pairs such as I go : I'm going ≠ I know : I'm knowing). Part of the meaning of any fragment of experience is its potentiality for being variably construed in time.