Halliday & Matthiessen (2014: 268):
The two kinds of Attribute differ in how they are realised: entity Attributes are realised by nominal groups with Thing as Head, e.g. architect in an architect; quality Attributes are realised by nominal groups with Epithet as Head, e.g. very generous. With the latter, the Thing is thus implicit; the general sense is ‘one’ – that is, the class of the Thing is presumed from the context.