This is a new feature in AR 6.1 which permits sharing application logic for models that are backed by multiple collections. For example, there are posts and comments that have similar attributes and thus similar logic, but they shouldn't be stored in the same collection (and thus STI wouldn't work for implementation).

https://guides.rubyonrails.org/6_1_release_notes.html#delegated-types

https://github.com/rails/rails/pull/39341