well my guess is that everybody who refers to 'upper and lower tier' is talking about US News rankings
As far as what the cutoffs are, who knows. I've heard a lot of people talk about top 20, top 10, top 5 programs according to US News and then refer to everything else as 'lower tier'
Disclaimer: I'm not advocating the US News rankings. Please reserve your "ranking *****" claims to MacGyver.