Halliday & Matthiessen (1999: 167):
We are using the term participanthood to suggest that the status of participant in the grammar is not absolute, but rather is a matter of degree. Among the various functional roles that the grammar construes as participants, we will discuss here two that are at opposite ends of the scale: Goal, which is has a clear status as prototypical participant, and Range, whose status as participant is much less clearly established.