I don't know about law school, but for Undergrad USNWR re-arranged the tiers so that Top Tier included, generally speaking, the Top-100. The next group was Tier-3 and that was up to 150 or 175. And then Tier-4 was the rest of schools.
I think they eliminated Tier-5 because they made Tier-3 bigger and just put the rest in Tier-4. They did that instead of separating each tier by 50.