Halliday & Matthiessen (1999: 211-2):
The most prototypical qualities are those of elaboration, subtype attribution: they stand in a purely intensive relation to the thing they are assigned to, being construed as inherent qualities. They are typically scalar, but certain types are taxonomic or binary.
As Epithets (or Classifiers) they are agnate with the Attribute of a figure of being. Many of them can serve as the Attribute of a figure of doing (as in he squashed it flat, she painted it blue) and are related to the outcome of such figures (cf. she heated/widened/enlarged it : it was hot/wide/large).
The limiting case of qualities of attribution are quantities; they are assigned to a thing as a discoursal instance rather than as a general experiential class. Consequently, they do not serve as Epithets; they have a special role, that of Numerative (as in two/many brave volunteers).