What is a sharing list?

I’m trying to wrap my head around Realm and the Rules section where first, I need to apply rules to collections. I have a “users” collection (people who sign up to use the application") and a, for the sake of this topic a “posts” collection.
During the Rules setup I see these template options. What is a “sharing list” and what would a possible use case be for it given my two collections?

@Andrew_W If you select that template the UI will display the configuration parameters you will need to set. In the sharing list case, each document will have an array field which contains the userIds for users which should be allowed to view that document.