@Robby zhu posted:
If I'm not mistaken, you might find what you're looking for here:
1. Sec 17.63 in CGEL by Quirk et al.
2. Chapter 12 sec 7.1 in the other CGEL.
It's just so happens that the grammar books you mentioned have the same acronym.
Indeed, I did not realise this coincidence! Thank you very much, Robby zhu, for these details! I will even add one additional section to No. 2 for future inquisitors:
2. Chapter 11, section 3.1 ("Conditions under which that must or may appear") (or, more specifically, pg. 953).