Halliday & Matthiessen (1999: 166):
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 and circumstances:(a) whether they are(i) general class,(ii) non-specific representatives of a class or(iii) specific representatives;(b) how far, and in what ways, they are involved in the process.