Halliday & Matthiessen (1999: 166-7):
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. We are using the term participanthood to suggest that the status of a participant in the grammar is not absolute, but rather a matter of degree. Among the various functional rôles that the grammar construes as participants, we will discuss here two that are at opposite ends of the scale: Goal, which has a clear status as prototypical participant, and Range, whose status as a participant is much less clearly established.