Not a BS family, but the impression I got from the colleges where this has been discussed is they typically will do whatever they feel like on an ad hoc basis.
Obviously if there are certain restricted majors, programs, schools, etc., then waitlist admission will depend on there being a slot available. But otherwise, they don’t tend to commit to some sort of strict ranking, and may, or may not, admit people based on whether their institutional goals have or have not yet been met with the enrolled class.
Kinda random, but Carnegie Mellon lets you choose between a “priority waitlist” and a “regular waitlist” and you are supposed to choose priority if CMU is your top college choice. It isn’t binding, but you are supposed to pay your deposit within 72 hours. In return for this, the idea is you will get a decision from CMU in the first few days after the enrollment deadline, before they go to the regular waitlist.
But they don’t guarantee everyone on the priority waitlist is ranked ahead of the regular waitlist, nor do they rank either waitlist internally. They just do what they want to do to fill out their programs as they see fit.
I think all that information is reflected in some form here:
That’s just CMU, of course, but so far at least my impression is a lot of other colleges similarly do not rank the waitlist. Here, for example, is Williams:
Relevant quote:
Are waiting list applicants ranked in any way?
No, there is no particular ordering of the waiting list. Instead, the Admission Committee periodically reconvenes to assess the composition of the enrolled class to see what additions might be made to round out the group.
And so on.