Question about the must hold rules in Locksmith

The built-in extension Locksmith defines a pair of rules called ‘the must hold the noun rule’ and ‘the must hold the second noun rule’.
However, I can’t see that they belong to any rulebook, nor that they are triggered by any other rule in that extension.
It’s plain what they do, but when are they meant to do it?

Good question. I’m curious now.