Halliday & Matthiessen (1999: 166):
While there is potential for variation, however, the variation is not arbitrary or random. The semantic system embodies certain general principles which guide the choice of one or other pattern of construal. These include:
process:(a) whether the process is non-actualised ('irrealis') or actualised ('realis');(b) how the process unfolds in time (its eventuation profile);participants & circumstances:(a) whether they are(i) general class,(ii) nonspecific representatives of a class or(iii) specific representatives;(b) how far, and in what ways, they are involved in the process.
We shall take the notion of degree of participanthood as an example of the general principle that the phenomena of experience may be construed as having more, or less, independent status within the semantic system.