Quiet narrowing is the failure this guards against: an ambiguous request, a defensible reading, and a deliverable that covers the easy half while appearing complete. Nobody is lied to and nobody can tell.
How to handle the ambiguity that produces the narrowing is a separate practice — PRACTICE-010, split out of this one — and the split is the point rather than tidiness: while both lived here, a boundary could not override one without the other.
When part of the scope turns out to be blocked, that part is named. Under VALUE-001, an unmentioned gap is a claim of completeness.