Example 1: This was the deepest a submarine had ever dived.
Example 2: The longest a person can hold their breath for is...
I've looked at a couple grammar resources including "the Cambridge grammar of the English Language", my issue is that I do not know the terminology required to find where this grammatical point would be explained. Superlative related entries do not explain this ellipted noun form, nor how it can be described in relation to the adjacent clause. I'm specifically looking to find a way to describe this grammatical construction so my ESL students can reproduce it. How would you formulate a 'rule' for its construction?