Halliday & Matthiessen (1999: 182):
We have already set up the general theory of participants, defining them in relation to ascriptive processes of being, being at, and having, in a process-participant configuration. There are three elements in such a configuration: the Process, intensive, possessive or circumstantial; the Attribute, which is being ascribed by one or other of these processes; and the Carrier. It is the rôle of Carrier which defines the concept of a participant. A participant, according to this theory, is that which may have assigned to it, in the discourse, properties [intensive/elaborating], parts [possessive/extending] or circumstantial features [enhancing].