Meaning of the new Generic Mail Avatars on iOS 18.2

Mail in the new iOS 18.2 assigns generic avatars to incoming emails from senders who do not appear in the recipient's Contacts.

This is separate from the new Categorized Mail feature, though the two features were released at the same time.

The generic avatars include: a white star on a pink background; a shopfront on a lavender background; a set of buildings on a pale blue background. I am sure there are more.

Does anyone know what each of these generic avatars is supposed to represent, and how Mail determines what avatar is assigned to which email? The generic avatars seem to have no relationship to Categorized Mail's four categories.

I've also noticed that the avatars sometimes change, at least for me. An e-mail that comes from "John Smith <john@smith.com>" might initially have a monogram avatar (i.e., a gray circular avatar with the white letters "JS"). But after a few minutes, it has one of the rectangular white-buildings-on-pale-blue generic avatars.

I know that next year, generic avatars will be replaced by brand logos for emails from senders that are part of Apple's Branded Mail (and maybe emails from senders using BIMI logos). But I'm guessing that the current generic avatars will stay in place for email from all other senders that are not in a recipient's Contact address book.

What does each of the generic avatars represent? How does Mail choose which avatar appears next to what email? Can I change the generic avatar that appears next to email from a particular sender or kind of sender?