I may be misremembering this, but I'm sure colleagues at some other schools have told me in the past that for P&T and/or appointments processes, external review letters are required to be from professors at U.S. News higher-ranked schools. If this is indeed correct, I'm interested in the genesis of the requirement - at any schools that have such a requirement. Presumably the actual "U.S. News rankings" aren't mentioned in any applicable school or university by-laws, so what does the language look like? Do schools have by-laws that require external referees to be at "peer schools" or is some other language used?
Also, wouldn't it make more sense to focus on the qualifications of the specific external reviewer to write the letter, rather than the school on whose faculty (s)he serves? Presumably a highly qualified and desirable external reviewer could be on the faculty at a lower ranked school or a poorly qualified reviewer - but nevertheless still teaching in the relevant field - could be on the faculty of a higher ranked school.
If there are schools that do have a "peer school" or "U.S. news higher ranked school" requirement for external reviewers, is the thought that they could comment on whether the candidate would qualify at their own school, thus giving the candidate's school comfort that a peer or higher-ranked school would consider the candidate for promotion/tenure/appointment? I know some schools do ask reviewers to comment on whether the candidate would achieve the promotion/appointment at their school and some ask reviewers to specifically refrain from making such comments. I've always wondered about that too. Surely the question is whether the applicant meets the standards as set out at his/her own institution, and it shouldn't be relevant if (s)he would qualify under another school's standards?
As the promotion and appointments season will shortly be upon us, I'm just musing on these issues and interested in others' thoughts ...