Halliday & Matthiessen (2014: 503):
A typical context for a nominal group with embedded enhancing clause is as Value in an identifying clause; cf. Figure 7-18. In this example the Token is also an embedded enhancing clause, this time functioning as Head. Such clauses often display a similar variation; for example:
Another reason is [[ that the quantity of the literature is not overwhelming yet]] .
Now the reason [[ they hired me]] is [[[ because they knew || I didn’t know anything about food]]] .
Identifying clauses of this kind, with nouns of expansion as the Head of the nominal group realising the Value, make an important contribution to the creation of discourse, making it possible to distribute information. Thus the textual impact of the time to leave is when people start to yawn is very different from that of you leave when people yawn: the former sets up the relationship as an exclusive identity, with the Value/Identified as Theme and the Identifier/Token as New (cf. the discussion of thematic equatives in Chapter 3, Section 3.2).