Halliday & Matthiessen (2014: 279):
In any ‘identifying’ clause, the two halves refer to the same thing; but the clause is not a tautology, so there must be some difference between them. This difference can be characterised as a stratal one of ‘expression’ and ‘content’; or, in terms of their generalised labels in the grammar, of Token and Value — and either can be used to identify the other.