Halliday & Matthiessen (1999: 207-8):
The Attribute is not a prototypical participant. We have already noted that as it stands it cannot function interpersonally as Subject. On the other hand, it can easily be instated as a participant by adding the noun or the noun substitute one(s) to the nominal group which realises it: this is a dry plate, this is a dry one. The fact that the thing can be instated as the Head of the nominal group serving as Attribute illustrates the point already made: the quality does not construe a separate class of thing, it presumes this class from the environment. Thus this is heavy means that it is heavy relatively to whatever class of thing it has been assigned to …