Halliday & Matthiessen (2014: 541-2):
In an ‘identifying’ clause, the fact clause serving as Token is identified with a Value realised by a nominal group with a noun as Head that typically belongs to the class of fact nouns; this fact noun may itself be qualified by an embedded fact clause. The Value is an interpretation of the fact clause, identifying it as a particular fact of some class of fact such as reason, problem, lesson, difficulty. … the Value nominal group may include an Epithet (thorniest, most important, plain … ; cf. obvious, indisputable, appalling, significant, simple, mere) assessing the fact represented by the Token (in the same way as Epithets within Attributes do) or a Numerative (cf. first, next, last). This latter is important in the development of discourse, being agnate with an internal temporal conjunction (cf. thirdly, the supreme interest for the whole world ...): the enumerated Value is the thematic point of departure of the clause and this Theme locates the clause as a message in the unfolding text.