Halliday & Matthiessen (2014: 122):
This system [THEME PREDICATION] resembles that of THEME IDENTIFICATION, in that it does identify one element as being exclusive at that point in the clause. Both are in fact equative constructions. But there are also differences between the two.Halliday & Matthiessen (2014: 122n):
Theme predication is often discussed under the heading of ‘cleft sentence’ – a term going back to Jespersen (e.g. 1928: 37, 88–92; 1937: Section 25.4), or ‘it-clefts’ to distinguish them from ‘wh- clefts’ or ‘pseudo-clefts’ (theme identification).