Halliday & Matthiessen (1999: 513):
… construing processes in this way clearly depends on generalising whole classes of phenomena; the grammar sets up classes of process, of participant, and of circumstance. There are various ways of doing this; one that is familiar in many languages is by means of a taxonomy of different kinds of word. The classes of word may be distinguished by their internal form, or by the way they are able to enter into larger constructions (or both).