Halliday & Matthiessen (2014: 376):
The Epithet indicates some quality of the subset, e.g. old, long, blue, fast; since qualities are denoted by adjectives, Epithets are often realised by adjectives. … (i) The quality of the subset may be an objective property of the thing itself, construed as a depiction of the experience of the entity that it represents; or (ii) it may be an expression of the speaker’s subjective attitude towards it, e.g. splendid, silly, fantastic. We refer to these as (i) experiential Epithets and (ii) interpersonal, or attitudinal Epithets, respectively. …
There is no hard and fast line between these two; but the former are experiential in function, whereas the latter, expressing the speaker’s attitude, represent an interpersonal element in the meaning of the nominal group (thus contributing to appraisal, in the sense of Martin & White, 2005).