Halliday & Matthiessen (1999: 207):
Typically, a quality combines with a thing to make up a participant in a figure: a dry plate, where the quality ‘dry’ is Epithet in the nominal group. The only context in which a quality serves on its own in a participant rôle is as Attribute; here it stands in intensive relation to a participant, either (i) in a figure of being, where the participant is Medium/Carrier (i.e. its sole function is to have the quality ascribed to it) or (ii) in a figure of doing, where the participant is Medium/Actor or Medium/Goal and the quality results from the doing; for example:being: The plate’s dry — I’ve made the plate dryNote that, in the doing figure, where I is Actor, the Attribute could be omitted: I’ve wiped the plate; in the being figure it cannot, since the process itself is one of ascription and the other participant, if present, is merely the ascriber.
doing: I’ve wiped the plate dry